Optical frequency control device, optical oscillation device, frequency conversion device, and radio wave generation device

ABSTRACT

An optical frequency control device includes: a detection circuit to receive first light including a first frequency, receive second light including a second frequency, modulate the first light with a local oscillation signal, and detect a differential beat signal between the frequency of sideband light included in the modulated first light and the second frequency; a light source control circuit to change the second frequency by frequency-dividing the differential beat signal with a first frequency division number, by frequency-dividing a reference signal with a second frequency division number, and by outputting a phase error signal indicating a phase difference between the frequency-divided differential beat signal and the frequency-divided reference signal; and a signal processing unit to set each of the first frequency division number and the second frequency division number according to the set value of a frequency difference between the first frequency and the second frequency.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT International Application No.PCT/JP2020/043013 filed on Nov. 18, 2020, which claims priority under 35U.S.C. § 119(a) to PCT International Application No. PCT/JP2019/045159filed in Japan on Nov. 19, 2019, all of which are hereby expresslyincorporated by reference into the present application.

TECHNICAL FIELD

The present invention relates to an optical frequency control devicethat changes a frequency included in light, an optical oscillationdevice including the optical frequency control device, a frequencyconversion device including the optical oscillation device, and a radiowave generation device including the optical oscillation device.

BACKGROUND ART

Patent Literature 1 below discloses an optical frequency control devicethat controls a frequency difference between output light of asemiconductor laser diode, which is controlled output light, andreference light. The optical frequency control device includes anoptical coupler that mixes output light of a semiconductor laser diodewith reference light to generate a first intermediate frequency signal,a first local oscillator capable of continuously changing an oscillationfrequency, a second local oscillator having a constant oscillationfrequency, and a mixer that mixes a first intermediate frequency signalwith an output signal of the first local oscillator to generate a secondintermediate frequency signal.

In addition, the optical frequency control device includes a phasecomparator that detects a phase difference between an output signal of asecond local oscillator and a second intermediate frequency signal andoutputs an error signal depending on the phase difference, and a controlcurrent injecting device that changes the oscillation frequency of thesemiconductor laser diode based on the error signal.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Laid-Open Publication No.    H3-120774

SUMMARY OF INVENTION Technical Problem

In the optical frequency control device disclosed in Patent Literature1, the first local oscillator controls a frequency difference betweencontrolled output light and reference light by changing the oscillationfrequency. However, in order for the optical frequency control device tobe able to change the frequency difference between the controlled outputlight and the reference light in a wide band, for example, a frequencysynthesizer capable of changing the oscillation frequency in a wide bandneeds to be used as a first local oscillator. The frequency synthesizercapable of changing a frequency difference in a wide band is implementedwith a filter or the like for suppressing occurrence of an unnecessaryfrequency, and the number of occurrence of unnecessary frequenciesincreases as a variable range of the frequency difference is wider.Therefore, in the frequency synthesizer capable of changing thefrequency difference in a wide band, as the variable range of thefrequency difference is wider, the circuit scale of the filter or thelike to be implemented becomes larger, and there is a problem that theoptical frequency control device becomes larger.

The present invention has been made to solve the above problems, and anobject thereof is to obtain an optical frequency control device thatdoes not require a frequency synthesizer.

Solution to Problem

An optical frequency control device according to the present inventionincludes: a detection circuit to receive first light including a firstfrequency from a first light source, receive second light including asecond frequency from a second light source, modulate the first lightwith a local oscillation signal oscillated by a first local oscillationsignal source, and detect a differential beat signal including adifferential frequency between a frequency of sideband light included inthe first light, which is modulated, and the second frequency; a lightsource control circuit to change the second frequency included in thesecond light oscillated by the second light source by frequency-dividinga differential beat signal detected by the detection circuit with afirst frequency division number, by frequency-dividing a referencesignal oscillated by a reference signal source with a second frequencydivision number, and by outputting, to the second light source, a phaseerror signal, which indicates a phase difference between thedifferential beat signal after the frequency division and the referencesignal after the frequency division number; and a signal processor toset each of a first frequency division number and a second frequencydivision number in accordance with a set value of a frequency differencebetween the first frequency division number and the second frequencydivision number

Advantageous Effects of Invention

According to the present invention, it is possible to change thefrequency difference between the first frequency included in the firstlight and the second frequency included in the second light withoutusing the frequency synthesizer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a configuration diagram illustrating an optical oscillationdevice 2 including an optical frequency control device 1 according to afirst embodiment.

FIG. 2 is a configuration diagram illustrating a first light source 11.

FIG. 3 is a configuration diagram illustrating a second light source 12.

FIG. 4 is a configuration diagram illustrating a PLL circuit 22.

FIG. 5 is a configuration diagram illustrating a signal processing unit24.

FIG. 6 is an explanatory diagram illustrating a first frequency f₁included in first light, a second frequency f₂ included in second light,and frequencies f₁−fm and f₁+fm of sideband light.

FIG. 7 is an explanatory view illustrating offset locking light.

FIG. 8 is a configuration diagram illustrating an optical oscillationdevice 2 including an optical frequency control device 1 according to asecond embodiment.

FIG. 9 is a configuration diagram illustrating a signal processing unit72.

FIG. 10 is a configuration diagram illustrating a signal processing unit81 of an optical frequency control device 1 according to a thirdembodiment.

FIG. 11 is an explanatory diagram illustrating a first frequency f₁included in first light, a second frequency f₂ included in second light,and respective frequencies of sideband light of orders d of −2 to +3.

FIG. 12 is a configuration diagram illustrating an optical oscillationdevice 2 including an optical frequency control device 1 according to afourth embodiment.

FIG. 13 is a configuration diagram illustrating a PLL circuit 91.

FIG. 14 is a configuration diagram illustrating a signal processing unit100.

FIG. 15 is an explanatory diagram illustrating a first frequency f₁included in first light, a second frequency f₂ included in second light,and respective frequencies of sideband light of orders d of −2 to +2.

FIG. 16 is a configuration diagram illustrating an optical oscillationdevice 2 including an optical frequency control device 1 according to afifth embodiment.

FIG. 17 is a configuration diagram illustrating a signal processing unit120.

FIG. 18 is an explanatory diagram illustrating a calculation example ofa sideband level with respect to modulation power of an LN phasemodulator 17.

FIG. 19 is a configuration diagram illustrating an optical oscillationdevice 2 including an optical frequency control device 1 according to asixth embodiment.

FIG. 20 is an explanatory diagram illustrating a first frequency f₁included in first light, frequencies f₁−fm and f₁+fm of sideband lightincluded in modulated first light, a second frequency f₂ included insecond light, and frequencies f₂−fn and f₂+fn of sideband light includedin modulated second light.

FIG. 21 is an explanatory view illustrating offset locking light.

FIG. 22 is a configuration diagram illustrating an optical oscillationdevice 2 including an optical frequency control device 1 according to aseventh embodiment.

FIG. 23 is an explanatory diagram illustrating a first frequency f₁included in first light, frequencies f₁−fm and f₁+fm of sideband lightincluded in modulated first light, a second frequency f₂ included insecond light, and frequencies f₂−fm and f₂+fm of sideband light includedin modulated second light.

FIG. 24 is an explanatory view illustrating offset locking light.

FIG. 25 is an explanatory diagram illustrating a first frequency f₁included in first light, frequencies f₁−fm, f₁+fm, f₁−2fm, and f₁+2fm ofsideband light included in the modulated first light, a second frequencyf₂ included in second light, and frequencies f₂−fm, f₂+fm, f₂−2fm, andf₂+2fm of sideband light included in the modulated second light.

FIG. 26 is an explanatory view illustrating offset locking light.

FIG. 27 is a configuration diagram illustrating an optical oscillationdevice 2 including an optical frequency control device 1 according to aneighth embodiment.

FIG. 28A is an explanatory diagram illustrating a change in sidebandlight when a frequency fm of a local oscillation signal is regulated tobe small, and FIG. 28B is an explanatory diagram illustrating a changein sideband light when the frequency fm of the local oscillation signalis regulated to be large.

FIG. 29 is a configuration diagram illustrating a frequency conversiondevice according to a ninth embodiment.

FIG. 30 is a configuration diagram illustrating a radio wave generationdevice according to a tenth embodiment.

FIG. 31 is an explanatory diagram illustrating an example in which allof a first light source 11, a second light source 12, and the detectioncircuit 16 are integrated as a planar lightwave circuit.

FIG. 32 is an explanatory diagram illustrating an example in which allof the first light source 11, the second light source 12, the detectioncircuit 16, a light source control circuit 21, and the signal processingunit 24 are integrated using silicon.

DESCRIPTION OF EMBODIMENTS

Hereinafter, in order to explain the present invention in more detail, amode for carrying out the present invention will be described based onthe accompanying drawings.

First Embodiment

FIG. 1 is a configuration diagram illustrating an optical oscillationdevice 2 including an optical frequency control device 1 according to afirst embodiment.

In FIG. 1 , the optical frequency control device 1 is a device capableof varying an offset frequency that is a frequency difference Δf(=f₂−f₁)between a first frequency f₁ included in first light and a secondfrequency f₁ included in second light.

The optical oscillation device 2 is a device, which includes the opticalfrequency control device 1, and outputs offset locking light that islight including the first frequency f₁ and the second frequency f₂.

A first light source 11 changes the first frequency f₁ included in thefirst light in accordance with a first control signal output from asignal processing unit 24 described later, and oscillates the firstlight including the changed first frequency f₁.

The first light source 11 is connected to a first optical distributor 13described later via, for example, an optical fiber, and outputs thefirst light including the first frequency f₁ to the first opticaldistributor 13.

A second light source 12 changes a second frequency f₂ included insecond light in accordance with each of a phase error signal output froma loop filter 23 described later included in a light source controlcircuit 21 described later and a second control signal output from thesignal processing unit 24, and oscillates the second light including thechanged second frequency f₂.

The second light source 12 is connected to a second optical distributor14 described later via, for example, an optical fiber, and outputs thesecond light including the second frequency f₂ to the second opticaldistributor 14.

The first optical distributor 13 distributes the first light oscillatedby the first light source 11.

The first optical distributor 13 is connected to an LN phase modulator17 described later and included in a detection circuit 16 describedlater via, for example, an optical fiber. LN is lithium niobate.

The first optical distributor 13 outputs first split light which is onelight after the distribution to the LN phase modulator 17, and outputsfirst synchronization light which is the other light after thedistribution to the outside of the device as one light in offset lockinglight.

The second optical distributor 14 distributes the second lightoscillated by the second light source 12.

The second optical distributor 14 is connected to an optical multiplexer18 described later included in the detection circuit 16 via, forexample, an optical fiber.

The second optical distributor 14 outputs second split light, which isone light after the distribution, to the optical multiplexer 18, andoutputs second synchronization light, which is the other light after thedistribution, to the outside of the device as the other light in theoffset locking light.

A first local oscillation signal source 15 oscillates a localoscillation signal having a frequency of fin.

The first local oscillation signal source 15 is connected to the LNphase modulator 17 via, for example, an optical fiber, and outputs alocal oscillation signal to the LN phase modulator 17.

The detection circuit 16 includes the LN phase modulator 17, the opticalmultiplexer 18, and a photodiode 19.

The detection circuit 16 receives the first light from the first lightsource 11 and receives the second light from the second light source 12.

The detection circuit 16 modulates the first light oscillated by thefirst light source 11 by the local oscillation signal oscillated by thefirst local oscillation signal source 15, and detects a differentialbeat signal including a differential frequency between the frequencyf₁+fm of sideband light included in the modulated first light and thesecond frequency f₂ included in the second light oscillated by thesecond light source 12.

The detection circuit 16 outputs the differential beat signal to thelight source control circuit 21.

The LN phase modulator 17 modulates the first split light output fromthe first optical distributor 13 by the local oscillation signaloscillated by the first local oscillation signal source 15, therebygenerating sideband light having a frequency of f₁±fm. fm is amodulation frequency by the LN phase modulator 17.

The LN phase modulator 17 is connected to the optical multiplexer 18via, for example, an optical fiber, and outputs modulation lightincluding each of the first frequency f₁ and the frequency f₁±fm to theoptical multiplexer 18 as the modulated first light.

In the optical frequency control device 1 illustrated in FIG. 1 , thedetection circuit 16 includes the LN phase modulator 17. However, theembodiment is not limited to the LN phase modulator 17 as long as themodulator can generate sideband light, and the detection circuit 16 mayinclude a Mach-Zehnder intensity modulator, an electro-absorptionmodulator, or the like.

The optical multiplexer 18 multiplexes the modulation light output fromthe LN phase modulator 17 and the second split light output from thesecond optical distributor 14.

The optical multiplexer 18 is connected to the photodiode 19 via, forexample, an optical fiber, and outputs the multiplexed light of themodulation light and the second split light to the photodiode 19.

The photodiode 19 converts the multiplexed light output from the opticalmultiplexer 18 into an electric signal.

The photodiode 19 detects, from the electric signals, a differentialfrequency f₂−(f₁+fm) between the second frequency f₂ included in thesecond light and the frequency f₁+fm of the sideband light.

The photodiode 19 outputs a signal including the differential frequencyf₂−(f₁+fm) as a differential beat signal to a phase locked loop (PLL)circuit 22 described later included in the light source control circuit21.

A reference signal source 20 oscillates a reference signal having afrequency fr.

The reference signal source 20 outputs the reference signal to the PLLcircuit 22.

The light source control circuit 21 includes the PLL circuit 22 and theloop filter 23.

The light source control circuit 21 frequency-divides the differentialbeat signal detected by the detection circuit 16 by a first frequencydivision number N.

The light source control circuit 21 frequency-divides the referencesignal oscillated by the reference signal source 20 by a secondfrequency division number R.

The light source control circuit 21 outputs a phase error signalindicating a phase difference between the frequency-divided differentialbeat signal and the frequency-divided reference signal to the secondlight source 12, thereby changing the second frequency f₂ included inthe second light oscillated by the second light source 12.

The PLL circuit 22 frequency-divides the differential beat signal outputfrom the photodiode 19 by the first frequency division number N.

The PLL circuit 22 frequency-divides the reference signal output fromthe reference signal source 20 by the second frequency division numberR.

The PLL circuit 22 outputs a phase error signal indicating a phasedifference between the frequency-divided differential beat signal andthe frequency-divided reference signal to the loop filter 23.

Note that the PLL circuit 22 may be an integral N-type PLL circuit or afractional N-type PLL circuit capable of setting fractional frequencydivision.

The loop filter 23 integrates the phase error signal output from the PLLcircuit 22 and outputs the integrated phase error signal to the secondlight source 12, thereby changing the second frequency f₂ included inthe second light oscillated by the second light source 12.

The signal processing unit 24 sets each of the first frequency divisionnumber N and the second frequency division number R in accordance with aset value Δf_(set) of a frequency difference Δf between the firstfrequency f₁ included in the first light and the second frequency f₂included in the second light.

The signal processing unit 24 outputs each of the first frequencydivision number N and the second frequency division number R to the PLLcircuit 22.

The signal processing unit 24 sets each of a first control signal forcontrolling the first frequency f₁ and a second control signal forcontrolling the second frequency f₂ in accordance with the set valueΔf_(set).

The signal processing unit 24 outputs the first control signal to thefirst light source 11 and outputs the second control signal to thesecond light source 12.

The first control signal includes, as a control signal for controlling alaser diode 31 (see FIG. 2 ), which will be described later, included inthe first light source 11, a set value of an injection current of thelaser diode 31.

Moreover, the first control signal includes a set value of an elementtemperature of the laser diode 31 as a control signal for controlling aPeltier element 35 (see FIG. 2 ) which is described later and includedin the first light source 11.

The second control signal includes a set value of an injection currentof a laser diode 41 as a control signal for controlling the laser diode41 (see FIG. 3 ) which is described later and included in the secondlight source 12.

Furthermore, the second control signal includes a set value of theelement temperature of the laser diode 41 as a control signal forcontrolling a Peltier element 45 (see FIG. 3 ) which is described laterand included in the second light source 12.

FIG. 2 is a configuration diagram illustrating the first light source11.

The first light source 11 includes the laser diode 31, a constantcurrent driver 32, a thermistor 33, a Thermo electric coolers (TEC)driver 34, and the Peltier element 35.

The laser diode 31 oscillates the first light and outputs the firstlight to the first optical distributor 13.

The constant current driver 32 adjusts the first frequency f₁ includedin the first light oscillated by the laser diode 31 by controlling theinjection current of the laser diode 31 in accordance with the set valueof the injection current of the laser diode 31, which is included in thefirst control signal output from the signal processing unit 24.

The thermistor 33 detects an element temperature of the laser diode 31and outputs temperature information indicating the element temperatureto the TEC driver 34.

The TEC driver 34 controls the current to be output to the Peltierelement 35 based on the difference between the set value of the elementtemperature of the laser diode 31 and the element temperature indicatedby the temperature information output from the thermistor 33, which isincluded in the first control signal output from the signal processingunit 24.

The Peltier element 35 adjusts the first frequency f₁ included in thefirst light oscillated by the laser diode 31 by controlling the elementtemperature of the laser diode 31 in accordance with the current outputfrom the TEC driver 34.

The first light source 11 illustrated in FIG. 2 includes the laser diode31, the constant current driver 32, the thermistor 33, the TEC driver34, and the Peltier element 35. However, the first light source 11 isnot limited to the configuration illustrated in FIG. 2 as long as thefirst frequency f₁ included in the first light can be adjusted. Forexample, the first light source 11 may be configured to control thefirst frequency f₁ included in the first light by controlling theresonator length of the light source, or may be configured to controlthe first frequency f₁ included in the first light by performing theserrodyne modulation and shifting the light source frequency.

FIG. 3 is a configuration diagram illustrating the second light source12.

The second light source 12 includes the laser diode 41, a constantcurrent driver 42, a thermistor 43, a TEC driver 44, and the Peltierelement 45.

The laser diode 41 oscillates the second light and outputs the secondlight to the second optical distributor 14.

The constant current driver 42 adjusts the set value of the injectioncurrent of the laser diode 41 included in the second control signaloutput from the signal processing unit 24 in accordance with the phaseerror signal after the integration output from the loop filter 23.

The constant current driver 42 adjusts the second frequency f₂ includedin the second light oscillated by the laser diode 41 by controlling theinjection current of the laser diode 41 in accordance with the adjustedset value.

The thermistor 43 detects an element temperature of the laser diode 41and outputs temperature information indicating the element temperatureto the TEC driver 44.

The TEC driver 44 controls the current to be output to the Peltierelement 45 based on the difference between the set value of the elementtemperature of the laser diode 41 and the element temperature indicatedby the temperature information output from the thermistor 43, which isincluded in the second control signal output from the signal processingunit 24.

The Peltier element 45 adjusts the second frequency f₂ included in thesecond light oscillated by the laser diode 41 by controlling the elementtemperature of the laser diode 41 in accordance with the current outputfrom the TEC driver 44.

The second light source 12 illustrated in FIG. 3 includes the laserdiode 41, the constant current driver 42, the thermistor 43, the TECdriver 44, and the Peltier element 45. However, the second light source12 is not limited to the configuration illustrated in FIG. 3 as long asthe second frequency f₂ included in the second light can be adjusted.For example, the second light source 12 may be configured to control thesecond frequency f₂ included in the second light by controlling theresonator length of the light source, or may be configured to controlthe second frequency f₂ included in the second light by performing theserrodyne modulation and shifting the light source frequency.

FIG. 4 is a configuration diagram illustrating the PLL circuit 22.

The PLL circuit 22 includes a prescaler 51, a prescaler 52, and a phasecomparator 53.

The prescaler 51 frequency-divides the differential beat signal outputfrom the photodiode 19 by the first frequency division number N outputfrom the signal processing unit 24.

The prescaler 51 outputs the frequency-divided differential beat signalto the phase comparator 53.

The prescaler 52 frequency-divides a reference signal output from thereference signal source 20 by the second frequency division number Routput from the signal processing unit 24.

The prescaler 52 outputs the frequency-divided reference signal to thephase comparator 53.

The phase comparator 53 detects a phase difference between thefrequency-divided differential beat signal output from the prescaler 51and the frequency-divided reference signal output from the prescaler 52.

The phase comparator 53 outputs a phase error signal indicating a phasedifference to the loop filter 23.

Note that the phase comparator 53 may be a current output type phasecomparator or a voltage output type phase comparator.

FIG. 5 is a configuration diagram illustrating the signal processingunit 24.

The signal processing unit 24 includes a frequency division numbersetting unit 61 and a control signal setting unit 62.

The frequency division number setting unit 61 includes a subtractor 61a, a divider 61 b, and a multiplier 61 c.

The internal memory of the frequency division number setting unit 61stores each of the modulation frequency fm, the frequency fr and thesecond frequency division number R.

In the signal processing unit 24 illustrated in FIG. 5 , each of themodulation frequency fm, the frequency fr and the second frequencydivision number R is stored in the internal memory of the frequencydivision number setting unit 61. However, this is merely an example, andeach of the modulation frequency fm, the frequency fr, and the secondfrequency division number R may be provided from the outside of thesignal processing unit 24.

For example, when a set value Δf_(set) of the frequency difference Δf isgiven from the outside of the device, the frequency division numbersetting unit 61 sets the first frequency division number N in accordancewith each of the set value Δf_(set), the modulation frequency fm, thefrequency fr, and the second frequency division number R.

The frequency division number setting unit 61 outputs each of the firstfrequency division number N and the second frequency division number Rto the PLL circuit 22.

For example, when a set value Δf_(set) of the frequency difference Δf isgiven from the outside of the device, the subtractor 61 a subtracts themodulation frequency fm from the set value Δf_(set), and outputs asubtraction result Δf_(set)−fm to the multiplier 61 c.

The divider 61 b divides the second frequency division number R by thefrequency fr and outputs a division result R/fr to the multiplier 61 c.

The multiplier 61 c multiplies the subtraction result Δf_(set)−fm outputfrom the subtractor 61 a by the division result R/fr output from thedivider 61 b, and outputs the multiplication result (Δf_(set)−fm)×R/frto the prescaler 51 of the PLL circuit 22 as the first frequencydivision number N.

The control signal setting unit 62 includes a table 62 a that storeseach of the first control signal and the second control signalcorresponding to the set value Δf_(set) of the frequency difference Δf.

Table 62 a stores, as first control signals, a set value of an injectioncurrent of the laser diode 31 corresponding to a set value Δf_(set) anda set value of an element temperature of the laser diode 31corresponding to a set value Δf_(set).

The table 62 a stores, as second control signals, a set value of aninjection current of the laser diode 41 corresponding to a set valueΔf_(set) and a set value of an element temperature of the laser diode 41corresponding to the set value Δf_(set).

The control signal setting unit 62 extracts each of the first controlsignal and the second control signal from the table 62 a, outputs thefirst control signal to the first light source 11, and outputs thesecond control signal to the second light source 12.

In the optical frequency control device 1 illustrated in FIG. 1 , one ormore of the first light source 11, the second light source 12, and thedetection circuit 16 are integrated as a planar lightwave circuit.

FIG. 31 is an explanatory diagram illustrating an example in which allof the first light source 11, the second light source 12, and thedetection circuit 16 are integrated as a planar lightwave circuit.

Furthermore, in the optical frequency control device 1 illustrated inFIG. 1 , one or more of the first light source 11, the second lightsource 12, the detection circuit 16, the light source control circuit21, and the signal processing unit 24 may be integrated using silicon.

FIG. 32 is an explanatory diagram illustrating an example in which allof the first light source 11, the second light source 12, the detectioncircuit 16, the light source control circuit 21, and the signalprocessing unit 24 are integrated using silicon.

Next, the operation of the optical oscillation device 2 illustrated inFIG. 1 will be described.

The first light source 11 changes the first frequency f₁ included in thefirst light in accordance with the first control signal output from thesignal processing unit 24, and oscillates the first light including thechanged first frequency f₁.

The first light source 11 outputs the first light including the firstfrequency f₁ to the first optical distributor 13.

The second light source 12 changes the second frequency f₂ included inthe second light in accordance with each of the phase error signaloutput from the loop filter 23 of the light source control circuit 21and the second control signal output from the signal processing unit 24,and oscillates the second light including the changed second frequencyf₂.

The second light source 12 outputs the second light including the secondfrequency f₂ to the second optical distributor 14.

When receiving the first light from the first light source 11, the firstoptical distributor 13 distributes the first light.

The first optical distributor 13 outputs the first split light, which isone light after the distribution, to the LN phase modulator 17.

The first optical distributor 13 outputs the first synchronizationlight, which is the other light after the distribution, to the outsideof the device as one light of the offset locking light.

When receiving the second light from the second light source 12, thesecond optical distributor 14 distributes the second light.

The second optical distributor 14 outputs the second split light, whichis one light after the distribution, to the optical multiplexer 18.

The second optical distributor 14 outputs the second synchronizationlight, which is the other light after the distribution, to the outsideof the device as the other light in the offset locking light.

The first local oscillation signal source 15 oscillates a localoscillation signal having a frequency of fm and outputs the localoscillation signal to the LN phase modulator 17.

The LN phase modulator 17 modulates the first split light output fromthe first optical distributor 13 by the local oscillation signaloscillated by the first local oscillation signal source 15, therebygenerating sideband light (see FIG. 6 ) having a frequency of f₁±fm. Thephase of the modulation frequency fm by the LN phase modulator 17 isstable, for example, in the microwave region.

The LN phase modulator 17 outputs the modulated light including each ofthe first frequency f₁ and the frequency f₁±fm to the opticalmultiplexer 18.

FIG. 6 is an explanatory diagram illustrating the first frequency f₁included in the first light, the second frequency f₂ included in thesecond light, and frequencies f₁−fm and f₁+fm of sideband light.

In FIG. 6 , f₂−(f₁+fm) is a differential frequency between the frequencyof f₂ included in the second light and the frequency f₁+fm of thesideband light.

In the optical frequency control device 1 illustrated in FIG. 1 , the LNphase modulator 17 generates sideband light having a frequency of f₁±fmat an interval of the modulation frequency fm on both sides of the firstfrequency f₁ with the first frequency f₁ as the center. However, this ismerely an example, and the LN phase modulator 17 may generate, forexample, only sideband light having a frequency of f₁+fm on one side ofthe first frequency f₁.

The optical multiplexer 18 multiplexes the modulation light output fromthe LN phase modulator 17 and the second split light output from thesecond optical distributor 14.

The optical multiplexer 18 outputs the multiplexed light of themodulated light and the second split light to the photodiode 19.

As illustrated in FIG. 6 , the multiplexed light includes a firstfrequency f₁ included in the first light, a second frequency f₂ includedin the second light, and frequencies f₁−fm and f₁+fm of the sidebandlight.

When receiving the multiplexed light from the optical multiplexer 18,the photodiode 19 converts the multiplexed light into an electricsignal.

The photodiode 19 detects, from the electric signals, a differentialfrequency f₂−(f₁+fm) between the second frequency f₂ included in thesecond light and the frequency f₁+fm of the sideband light.

The photodiode 19 outputs a signal including the differential frequencyf₂−(f₁+fm) to the PLL circuit 22 as a differential beat signal.

In the optical frequency control device 1 illustrated in FIG. 1 ,assuming that the first frequency f₁<the second frequency f₂, thephotodiode 19 detects the differential frequency f₂−(f₁+fm) between thesecond frequency f₂ included in the second light and the frequency f₁+fmof the sideband light from the electric signals. However, this is merelyan example, and for example, when the first frequency f₁>the secondfrequency f₂, the photodiode 19 may detect the differential frequency(f₁−fm)−f₂ between the second frequency f₂ included in the second lightand the frequency f₁−fm of the sideband light from the electric signals.In this case, the photodiode 19 outputs a signal including adifferential frequency (f₁−fm)−f₂ to the PLL circuit 22 as adifferential beat signal.

The reference signal source 20 oscillates a reference signal having afrequency fr and outputs the reference signal to the PLL circuit 22.

The light source control circuit 21 frequency-divides the differentialbeat signal output from the photodiode 19 by a first frequency divisionnumber N and frequency-divides the reference signal output from thereference signal source 20 by a second frequency division number R.

The light source control circuit 21 outputs a phase error signalindicating a phase difference between the frequency-divided differentialbeat signal and the frequency-divided reference signal to the secondlight source 12, thereby changing the second frequency f₂ included inthe second light oscillated by the second light source 12.

Hereinafter, the operation of the light source control circuit 21 willbe described in detail.

The prescaler 51 of the PLL circuit 22 frequency-divides thedifferential beat signal output from the photodiode 19 by the firstfrequency division number N output from the signal processing unit 24.

Since the frequency included in the differential beat signal isf₂−(f₁+fm), the frequency of the differential beat signal after thefrequency division by the prescaler 51 is (f₂−(f₁+fm))/N.

The prescaler 51 outputs the frequency-divided differential beat signalto the phase comparator 53.

The prescaler 52 of the PLL circuit 22 frequency-divides the referencesignal output from the reference signal source 20 by the secondfrequency division number R output from the signal processing unit 24.

Since the frequency included in the reference signal is fr, thereference signal after the frequency division by the prescaler 52 isfr/R.

The prescaler 52 outputs the frequency-divided reference signal to thephase comparator 53.

The phase comparator 53 of the PLL circuit 22 detects a phase differencebetween the frequency-divided differential beat signal output from theprescaler 51 and the frequency-divided reference signal output from theprescaler 52.

The phase comparator 53 outputs a phase error signal indicating a phasedifference to the loop filter 23.

The loop filter 23 integrates the phase error signal output from thephase comparator 53 and outputs the integrated phase error signal to thesecond light source 12.

When the phase error signal after the integration is output to thesecond light source 12, the second frequency f₂ included in the secondlight output from the second light source 12 changes.

When the phase difference detected by the phase comparator 53 convergesand phase synchronization of the PLL circuit 22 is fulfilled, thefollowing Equation (1) is established.

$\begin{matrix}{\frac{f_{2} - \left( {f_{1} + {fm}} \right)}{N} = \frac{fr}{R}} & (1)\end{matrix}$

Expression (1) can be organized as the following Expression (2).

$\begin{matrix}{{f_{2} - f_{1}} = {{\frac{N}{R}{fr}} + {fm}}} & (2)\end{matrix}$

Expression (2) represents an offset frequency f₂−f₁ between the firstfrequency f₁ included in the first light oscillated by the first lightsource 11 and the second frequency f₂ included in the second lightoscillated by the second light source 12. The offset frequency f₂−f₁ isdetermined by the first frequency division number N, the secondfrequency division number R, the frequency fr included in the referencesignal and the modulation frequency fm.

The offset frequency f₂−f₁ corresponds to a frequency difference betweenthe first frequency f₁ included in the first synchronization light andthe second frequency f₂ included in the second synchronization light.The first synchronization light and the second synchronization light arelight constituting offset locking light.

When Expression (1) is solved for N, the following Expression (3) isobtained.

$\begin{matrix}\begin{matrix}{N = {\frac{R}{fr}\left( {f_{2} - f_{1} - {fm}} \right)}} \\{= {\frac{R}{fr}\left( {{\Delta\; f_{set}} - {fm}} \right)}}\end{matrix} & (3)\end{matrix}$

Upon receiving the set value Δf_(set) of the frequency difference Δfbetween the first frequency f₁ and the second frequency f₂ from theoutside of the device, the signal processing unit 24 sets each of thefirst frequency division number N and the second frequency divisionnumber R in accordance with the set value Δf_(set).

The signal processing unit 24 outputs each of the first frequencydivision number N and the second frequency division number R to the PLLcircuit 22.

Moreover, the signal processing unit 24 sets each of the first controlsignal for controlling the first frequency f₁ and the second controlsignal for controlling the second frequency f₂ in accordance with theset value Δf_(set).

The signal processing unit 24 outputs the first control signal to thefirst light source 11 and outputs the second control signal to secondlight source 12.

Hereinafter, the operation of the signal processing unit 24 will bedescribed in detail.

First, the frequency division number setting unit 61 outputs the secondfrequency division number R stored in the internal memory to theprescaler 52 of the PLL circuit 22.

For example, when a set value Δf_(set) of the frequency difference Δf isgiven from the outside of the device, the subtractor 61 a of thefrequency division number setting unit 61 acquires the modulationfrequency fm stored in the internal memory, and subtracts the modulationfrequency fm from the set value Δf_(set).

The subtractor 61 a outputs the subtraction result Δf_(set)−fm to themultiplier 61 c.

The divider 61 b of the frequency division number setting unit 61acquires the second frequency division number R stored in the internalmemory and the frequency fr included in the reference signal stored inthe internal memory.

The divider 61 b divides the second frequency division number R by thefrequency fr and outputs a division result R/fr to the multiplier 61 c.

The multiplier 61 c of the frequency division number setting unit 61multiplies the subtraction result Δf_(set)−fm output from the subtractor61 a by the division result R/fr output from the divider 61 b.

The multiplication result (Δf_(set)−fm)×R/fr of the subtraction resultΔf_(set)−fm and the division result R/fr corresponds to the firstfrequency division number N from Expression (3).

The multiplier 61 c outputs the multiplication result (Δf_(set)−fm)×R/frto the prescaler 51 of the PLL circuit 22 as the first frequencydivision number N.

For example, when the set value Δf_(set) of the frequency difference Δfis given from the outside of the device, the control signal setting unit62 acquires the first control signal corresponding to the set valueΔf_(set) and the second control signal corresponding to the set valueΔf_(set) from the table 62 a.

That is, the control signal setting unit 62 acquires, as the firstcontrol signals from the table 62 a, the set value of the injectioncurrent of the laser diode 31 corresponding to the set value Δf_(set)and the set value of the element temperature of the laser diode 31corresponding to the set value Δf_(set).

Moreover, the control signal setting unit 62 acquires, as the secondcontrol signals from the table 62 a, the set value of the injectioncurrent of the laser diode 41 corresponding to the set value Δf_(set)and the set value of the element temperature of the laser diode 41corresponding to the set value Δf_(set).

The control signal setting unit 62 outputs the first control signal tothe first light source 11 and outputs the second control signal to thesecond light source 12.

In the table 62 a illustrated in FIG. 5 , the set value of the injectioncurrent of the laser diode 31 is described as the injection current setvalue of the first light source, and the set value of the elementtemperature of the laser diode 31 is described as the TEC set value ofthe first light source.

Furthermore, the set value of the injection current of the laser diode41 is described as the injection current set value of the second lightsource, and the set value of the element temperature of the laser diode41 is described as the TEC set value of the second light source.

In the table 62 a illustrated in FIG. 5 , when the set value Δf_(set) isΔf₁, the set value of the injection current of the laser diode 31 is 120mA, the set value of the element temperature of the laser diode 31 is30° C., the set value of the injection current of the laser diode 41 is100 mA, and the set value of the element temperature of the laser diode41 is 30° C.

Moreover, when the set value Δf_(set) is Δf₂, the set value of theinjection current of the laser diode 31 is 120 mA, the set value of theelement temperature of the laser diode 31 is 35° C., the set value ofthe injection current of the laser diode 41 is 110 mA, and the set valueof the element temperature of the laser diode 41 is 30° C.

When receiving the first control signal from the signal processing unit24, the constant current driver 32 of the first light source 11 extractsthe set value of the injection current of the laser diode 31 included inthe first control signal.

The constant current driver 32 adjusts the first frequency f₁ includedin the first light oscillated by the laser diode 31 by controlling theinjection current of the laser diode 31 in accordance with the set valueof the injection current.

The thermistor 33 of the first light source 11 detects the elementtemperature of the laser diode 31 and outputs temperature informationindicating the element temperature to the TEC driver 34.

Upon receiving the first control signal from the signal processing unit24, the TEC driver 34 extracts the set value of the element temperatureof the laser diode 31 included in the first control signal.

The TEC driver 34 controls the current to be output to the Peltierelement 35 based on the difference between the set value of the elementtemperature of the laser diode 31 and the element temperature indicatedby the temperature information output from the thermistor 33.

The Peltier element 35 adjusts the first frequency f₁ included in thefirst light oscillated by the laser diode 31 by controlling the elementtemperature of the laser diode 31 in accordance with the current outputfrom the TEC driver 34.

Thus, the first frequency f₁ included in the first light is adjusted inaccordance with the set value Δf_(set) of the frequency difference Δf.

When receiving the second control signal from the signal processing unit24, the constant current driver 42 of the second light source 12extracts the set value of the injection current of the laser diode 41included in the second control signal.

The constant current driver 42 adjusts the set value of the injectioncurrent of the laser diode 41 in accordance with the integrated phaseerror signal output from the loop filter 23. The constant current driver42 adjusts the set value of the injection current, for example, bysubtracting the phase error signal after the integration from the setvalue of the injection current.

The constant current driver 42 adjusts the second frequency f₂ includedin the second light oscillated by the laser diode 41 by controlling theinjection current of the laser diode 41 in accordance with the adjustedset value.

The thermistor 43 of the second light source 12 detects the elementtemperature of the laser diode 41 and outputs temperature informationindicating the element temperature to the TEC driver 44.

Upon receiving the second control signal from the signal processing unit24, the TEC driver 44 extracts the set value of the element temperatureof the laser diode 41 included in the second control signal.

The TEC driver 44 controls the current to be output to the Peltierelement 45 based on the difference between the set value of the elementtemperature of the laser diode 41 and the element temperature indicatedby the temperature information output from the thermistor 43.

The Peltier element 45 adjusts the second frequency f₂ included in thesecond light oscillated by the laser diode 41 by controlling the elementtemperature of the laser diode 41 in accordance with the current outputfrom the TEC driver 44.

Thus, the second frequency f₂ included in the second light is adjustedin accordance with the set value Δf_(set) of the frequency differenceΔf.

When receiving the first light from the first light source 11, the firstoptical distributor 13 distributes the first light and outputs the firstsplit light, which is one light after the distribution, to the LN phasemodulator 17.

The first optical distributor 13 outputs the first synchronizationlight, which is the other light after the distribution, to the outsideof the device as one light of the offset locking light.

When receiving the second light from the second light source 12, thesecond optical distributor 14 distributes the second light and outputsthe second split light, which is one of the distributed light, to theoptical multiplexer 18.

The second optical distributor 14 outputs the second synchronizationlight, which is the other light after the distribution, to the outsideof the device as the other light in the offset locking light.

FIG. 7 is an explanatory diagram illustrating offset locking light.

In FIG. 7 , the frequency f₁ is the first frequency included in thefirst synchronous light, and the frequency f₂ is the second frequencyincluded in the second synchronous light.

Since each of the first frequency f₁ and the second frequency f₂ isadjusted in accordance with the set value Δf_(set) of the frequencydifference Δf, the frequency difference Δf between the first frequencyf₁ and the second frequency f₂ can be changed.

In the first embodiment described above, the optical frequency controldevice 1 is configured to include the detection circuit 16 to receivethe first light including the first frequency from the first lightsource 11, receive the second light including the second frequency fromthe second light source 12, modulate the first light with the localoscillation signal oscillated by the first local oscillation signalsource 15, and detect the differential beat signal including thedifferential frequency between the frequency of the sideband lightincluded in the modulated first light and the second frequency, thelight source control circuit 21 to change the second frequency includedin the second light oscillated by the second light source 12 byfrequency-dividing the differential beat signal detected by thedetection circuit 16 with the first frequency division number, byfrequency-dividing the reference signal oscillated by the referencesignal source 20 with the second frequency division number, and byoutputting the phase error signal indicating the phase differencebetween the divided differential beat signal and the divided referencesignal to the second light source 12, and the signal processing unit 24to set each of the first frequency division number and the secondfrequency division number according to the set value of the frequencydifference between the first frequency and the second frequency.Therefore, the optical frequency control device 1 can change thefrequency difference between the first frequency included in the firstlight and the second frequency included in the second light withoutusing the frequency synthesizer.

Second Embodiment

In a second embodiment, an optical frequency control device 1, in whicha control signal setting unit 75 adjusts each of a first control signaland a second control signal by updating an internal table 62 a on thebasis of the phase error signal output from the light source controlcircuit 21, will be described.

FIG. 8 is a configuration diagram illustrating an optical oscillationdevice 2 including an optical frequency control device 1 according tothe second embodiment. In FIG. 8 , the same reference signs as those inFIG. 1 denote the same or corresponding parts, and thus the descriptionthereof is omitted.

A voltage monitor 71 samples a phase error signal after the integrationoutput from a loop filter 23 and converts the sampled phase error signalinto voltage data.

The voltage monitor 71 outputs the voltage data to the signal processingunit 72 described later.

Similarly to the signal processing unit 24 illustrated in FIG. 1 , thesignal processing unit 72 sets each of a first frequency division numberN and a second frequency division number R in accordance with a setvalue Δf_(set) of a frequency difference Δf between a first frequency f₁included in first light and a second frequency f₂ included in secondlight.

The signal processing unit 72 outputs each of the first frequencydivision number N and the second frequency division number R to a PLLcircuit 22.

Similarly to the signal processing unit 24 illustrated in FIG. 1 , thesignal processing unit 72 sets each of a first control signal forcontrolling the first frequency f₁ and a second control signal forcontrolling the second frequency f₂ in accordance with the set valueΔf_(set).

The signal processing unit 72 outputs a first control signal to a firstlight source 11 and outputs a second control signal to a second lightsource 12.

Unlike the signal processing unit 24 illustrated in FIG. 1 , the signalprocessing unit 72 adjusts each of the first control signal and thesecond control signal on the basis of the voltage data output from thevoltage monitor 71.

FIG. 9 is a configuration diagram illustrating the signal processingunit 72.

Similarly to the signal processing unit 24 illustrated in FIG. 5 , thesignal processing unit 72 illustrated in FIG. 9 includes a frequencydivision number setting unit 61.

Unlike the signal processing unit 24 illustrated in FIG. 5 , the signalprocessing unit 72 illustrated in FIG. 9 includes a voltage rangestoring unit 73, a comparator 74 and a control signal setting unit 75.

The voltage range storing unit 73 is a storage medium for storing thevoltage range of the phase error signal after the integration outputfrom the loop filter 23.

In a case where the voltage swing width of the phase error signal afterthe integration output from the loop filter 23 is designed to be in therange of 0 to 10 V, for example, a range of 3 to 7 V narrower than thevoltage swing width, for example, is stored in the voltage range storingunit 73 as the voltage range of the phase error signal.

When the voltage data output from the voltage monitor 71 is out of therange of the voltage range stored in the voltage range storing unit 73,the comparator 74 updates a table 62 a included in the control signalsetting unit 75.

Similarly to the control signal setting unit 62 illustrated in FIG. 5 ,the control signal setting unit 75 includes the table 62 a that storeseach of the first control signal and the second control signalcorresponding to the set value Δf_(set) of the frequency difference Δf.

Similarly to the control signal setting unit 62 illustrated in FIG. 5 ,the control signal setting unit 75 takes each of the first controlsignal and the second control signal out from the table 62 a, outputsthe first control signal to the first light source 11, and outputs thesecond control signal to the second light source 12.

The table 62 a included in the control signal setting unit 75 is updatedby the comparator 74 unlike the table 62 a included in the controlsignal setting unit 62 illustrated in FIG. 5 .

Next, the operation of the optical frequency control device 1illustrated in FIG. 8 will be described.

The optical frequency control device 1 illustrated in FIG. 8 operates ina way that, when the voltage swing width of the phase error signal afterthe integration output from the loop filter 23 is designed to be in therange of 0 to 10 V, for example, the voltage range of the phase errorsignal is drawn into the range of 3 to 7 V, for example.

Since the components other than the voltage monitor 71 and the signalprocessing unit 72 are similar to those of the optical frequency controldevice 1 illustrated in FIG. 1 , only the operations of the voltagemonitor 71 and the signal processing unit 72 will be described herein.

The voltage monitor 71 samples a phase error signal after theintegration output from the loop filter 23 and converts the sampledphase error signal into voltage data.

The voltage monitor 71 outputs the voltage data to the comparator 74 ofthe signal processing unit 72.

The comparator 74 compares the voltage data output from the voltagemonitor 71 with the voltage range stored in the voltage range storingunit 73.

When the comparison result indicates that the voltage data is out of thevoltage range, the comparator 74 updates the table 62 a included in thecontrol signal setting unit 75.

Hereinafter, an example of updating the table 62 a by the comparator 74will be described in detail.

When the voltage range of the phase error signal is, for example, in therange of 3 to 7 V, if the voltage data output from the voltage monitor71 is smaller than 3 V, which is the lower limit of the voltage range,the comparator 74 updates the set values of the element temperatures oflaser diodes 31 and 41 stored in the table 62 a to be increased by, forexample, 1° C.

When the voltage data output from the voltage monitor 71 is larger thanthe upper limit 7 V of the voltage range, the comparator 74 updates theset values of the element temperatures of the laser diodes 31 and 41stored in the table 62 a to be lowered by, for example, 1° C.

If the voltage data output from the voltage monitor 71 is within thevoltage range, the comparator 74 does not update the set values of theelement temperatures of the laser diodes 31 and 41 stored in the table62 a.

By the comparator 74 updating the set value of the element temperaturestored in the table 62 a, the voltage range of the phase error signalfalls within the range of 3 to 7 V, for example.

In the optical frequency control device 1 illustrated in FIG. 8 , thecomparator 74 updates the set values of the element temperatures of thelaser diodes 31 and 41. However, it is sufficient that the voltage rangeof the phase error signal is updated so as to be fallen within, forexample, the range of 3 to 7 V and the comparator 74 may update the setvalue of the injection current of the laser diodes 31 and 41 stored inthe table 62 a, for example.

The control signal setting unit 75 takes out the first control signalcorresponding to the set value Δf_(set) and the second control signalcorresponding to the set value Δf_(set) from the table 62 a updated bythe comparator 74.

The control signal setting unit 75 outputs the first control signal tothe first light source 11 and outputs the second control signal to thesecond light source 12.

In the second embodiment described above, the optical frequency controldevice 1 illustrated in FIG. 8 is configured in such a manner that thesignal processing unit 72 adjusts each of the first control signal andthe second control signal on the basis of the phase error signal outputfrom the light source control circuit 21. Therefore, similarly to theoptical frequency control device 1 illustrated in FIG. 1 , the opticalfrequency control device 1 illustrated in FIG. 8 can change thefrequency difference between the first frequency included in the firstlight and the second frequency included in the second light withoutusing the frequency synthesizer.

Moreover, even if the oscillation frequencies of the first light source11, the second light source 12, and the first local oscillation signalsource 15 drift due to a change in the operating environment, a secularchange, or the like, the optical frequency control device 1 illustratedin FIG. 8 can avoid falling into lock-off during non-establishment ofphase synchronization or synchronization.

Furthermore, even when the voltage swing width of the phase error signaloutput from the loop filter 23 is narrow, it is possible to avoidfalling into lock-off. Thus, even when a light source with low phasenoise is used as the first light source 11, the second light source 12,and the first local oscillation signal source 15, synchronization can bemaintained, and low phase noise can be reduced.

Third Embodiment

In a third embodiment, an optical frequency control device 1, in which asignal processing unit 81 determines the order d of the sideband lighton the basis of a set value Δf_(set) and sets a first frequency divisionnumber N in accordance with each of the set value Δf_(set) and the orderd, will be described.

FIG. 10 is a configuration diagram illustrating the signal processingunit 81 of the optical frequency control device 1 according to the thirdembodiment. In FIG. 10 , the same reference signs as those in FIGS. 5and 9 denote the same or corresponding parts, and thus the descriptionthereof is omitted.

The signal processing unit 81 illustrated in FIG. 10 includes a voltagerange storing unit 73, a comparator 74, and a control signal settingunit 75, similarly to the signal processing unit 72 illustrated in FIG.9 . However, this is merely an example, and the signal processing unit81 may include the control signal setting unit 62 illustrated in FIG. 5instead of the voltage range storing unit 73, the comparator 74 and thecontrol signal setting unit 75.

A frequency division number setting unit 82 includes a subtractor 82 a,a comparator 82 b, an increment unit 82 c, a multiplier 82 d, a divider82 e, and a multiplier 82 f.

The internal memory of the frequency division number setting unit 82stores each of a modulation frequency fm, a frequency fr, and a secondfrequency division number R.

In the signal processing unit 81 illustrated in FIG. 10 , each of themodulation frequency fm, the frequency fr and the second frequencydivision number R is stored in the internal memory of the frequencydivision number setting unit 82. However, this is merely an example, andeach of the modulation frequency fm, the frequency fr, and the secondfrequency division number R may be given from the outside of the signalprocessing unit 81.

For example, when a set value Δf_(set) of a frequency difference Δf isgiven from the outside of the device, the frequency division numbersetting unit 82 determines the order d of the sideband light based oneach of the set value Δf_(set) and the modulation frequency fm.

The frequency division number setting unit 82 sets the first frequencydivision number N in accordance with each of the set value Δf_(set), theorder d, the modulation frequency fm, the frequency fr, and the secondfrequency division number R.

The frequency division number setting unit 82 outputs each of the firstfrequency division number N and the second frequency division number Rto the PLL circuit 22.

For example, when the set value Δf_(set) of the frequency difference Δfis given from the outside of the device, the subtractor 82 a subtractsthe multiplication result d×fm of the multiplier 83 d from the set valueΔf_(set), and outputs the subtraction result Δf_(set)−d×fm to thecomparator 82 b.

The comparator 82 b compares the subtraction result Δf_(set)−d×fm outputfrom the subtractor 82 a with the modulation frequency fm.

If the subtraction result Δf_(set)−d×fm is equal to or greater than themodulation frequency fm, the comparator 82 b outputs an incrementcommand of the order d to the increment unit 82 c.

When the subtraction result Δf_(set)−d×fm is smaller than the modulationfrequency fm, the comparator 82 b determines the order d at the presenttime as the order d of the sideband light and outputs the subtractionresult Δf_(set)−d×fm to the multiplier 82 f.

When receiving the increment command from the comparator 82 b, theincrement unit 82 c increments the order d by 1. The initial value ofthe order d is 0.

The multiplier 82 d multiplies the order d output from the incrementunit 82 c by the modulation frequency fm and outputs a multiplicationresult d×fm of the order d by the modulation frequency fm to thesubtractor 82 a.

The divider 82 e divides the second frequency division number R by thefrequency fr and outputs the division result R/fr to the multiplier 82f.

The multiplier 82 f multiplies the subtraction result Δf_(set)−d×fmoutput from the comparator 82 b by the division result R/fr output fromthe divider 82 e and outputs the multiplication result(Δf_(set)−d×fm)×R/fr to the prescaler 51 of the PLL circuit 22 as afirst frequency division number N.

Next, the operation of the signal processing unit 81 will be described.Note that, since the operation other than the frequency division numbersetting unit 82 is similar to that of the signal processing unit 72illustrated in FIG. 9 , only the operation of the frequency divisionnumber setting unit 82 will be described here.

First, the frequency division number setting unit 82 outputs the secondfrequency division number R stored in the internal memory to theprescaler 52 of the PLL circuit 22.

For example, when a set value Δf_(set) of the frequency difference Δf isgiven from the outside of the device, the subtractor 82 a subtracts themultiplication result d×fm of the multiplier 83 d from the set valueΔf_(set). Note that, since the initial value of the order d is 0, themultiplication result d×fm at the present time is 0.

The subtractor 82 a outputs the subtraction result Δf_(set)−d×fm to thecomparator 82 b.

The comparator 82 b compares the subtraction result Δf_(set)−d×fm outputfrom the subtractor 82 a with, for example, the modulation frequency fmstored in the internal memory.

If the subtraction result Δf_(set)−d×fm is equal to or greater than themodulation frequency fm, the comparator 82 b outputs an incrementcommand of the order d to the increment unit 82 c.

When the subtraction result Δf_(set)−d×fm is smaller than the modulationfrequency fm, the comparator 82 b outputs the subtraction resultΔf_(set)−d×fm to the multiplier 82 f.

Herein, for convenience of explanation, it is assumed that thesubtraction result Δf_(set)−d×fm is equal to or greater than themodulation frequency fm, and the comparator 82 b outputs an incrementcommand of the order d to the increment unit 82 c.

When receiving the increment command from the comparator 82 b, theincrement unit 82 c increments the order d by 1. Since the order dbefore being incremented is 0, the order d output from the incrementunit 82 c is 1.

The multiplier 82 d multiplies the order d output from the incrementunit 82 c by the modulation frequency fm and outputs a multiplicationresult d×fm of the order d by the modulation frequency fm to thesubtractor 82 a.

Upon receiving the multiplication result d×fm from the multiplier 82 d,the subtractor 82 a subtracts the multiplication result d×fm from theset value Δf_(set) of the frequency difference Δf. Since the order doutput from the increment unit 82 c is 1, the multiplication result d×fmis equal to fm.

The subtractor 82 a outputs the subtraction result Δf_(set)−d×fm to thecomparator 82 b.

The comparator 82 b compares the subtraction result Δf_(set)−d×fm outputfrom the subtractor 82 a with the modulation frequency fm.

If the subtraction result Δf_(set)−d×fm is equal to or greater than themodulation frequency fm, the comparator 82 b outputs an incrementcommand of the order d to the increment unit 82 c.

When the subtraction result Δf_(set)−d×fm is smaller than the modulationfrequency fm, the comparator 82 b outputs the subtraction resultΔf_(set)−d×fm to the multiplier 82 f.

Herein, for convenience of explanation, it is assumed that thesubtraction result Δf_(set)−d×fm is equal to or greater than themodulation frequency fm, and the comparator 82 b outputs an incrementcommand of the order d to the increment unit 82 c.

When receiving the increment command from the comparator 82 b, theincrement unit 82 c increments the order d by 1. Since the order dbefore being incremented is 1, the order d output from the incrementunit 82 c is 2.

The multiplier 82 d multiplies the order d output from the incrementunit 82 c by the modulation frequency fm and outputs a multiplicationresult d×fm of the order d by the modulation frequency fm to thesubtractor 82 a.

Upon receiving the multiplication result d×fm from the multiplier 82 d,the subtractor 82 a subtracts the multiplication result d×fm from theset value Δf_(set) of the frequency difference Δf. Since the order doutput from the increment unit 82 c is 2, the multiplication result d×fmis equal to 2fm.

The subtractor 82 a outputs the subtraction result Δf_(set)−d×fm to thecomparator 82 b.

The comparator 82 b compares the subtraction result Δf_(set)−d×fm outputfrom the subtractor 82 a with the modulation frequency fm.

If the subtraction result Δf_(set)−d×fm is equal to or greater than themodulation frequency fm, the comparator 82 b outputs an incrementcommand of the order d to the increment unit 82 c.

When the subtraction result Δf_(set)−d×fm is smaller than the modulationfrequency fm, the comparator 82 b outputs the subtraction resultΔf_(set)−d×fm to the multiplier 82 f.

Herein, for convenience of description, it is assumed that thesubtraction result Δf_(set)−d×fm is smaller than the modulationfrequency fm, and the comparator 82 b outputs the subtraction resultΔf_(set)−d×fm to the multiplier 82 f.

The divider 82 e acquires the second frequency division number R storedin the internal memory and the frequency fr included in the referencesignal stored in the internal memory.

The divider 82 e divides the second frequency division number R by thefrequency fr and outputs the division result R/fr to the multiplier 82f.

The multiplier 82 f multiplies the subtraction result Δf_(set)−d×fmoutput from the comparator 82 b by the division result R/fr output fromthe divider 82 e.

The multiplication result (Δf_(set)−d×fm)×R/fr of the subtraction resultΔf_(set)−d×fm and the division result R/fr corresponds to the firstfrequency division number N as shown in the following Expression (4).

The multiplier 61 c outputs the multiplication result(Δf_(set)−d×fm)×R/fr to the prescaler 51 of the PLL circuit 22 as thefirst frequency division number N.

$\begin{matrix}{N = {\frac{R}{fr}\left( {{\Delta\; f_{set}} - {d \times {fm}}} \right)}} & (4)\end{matrix}$

FIG. 11 is an explanatory diagram illustrating the first frequency f₁included in the first light, the second frequency f₂ included in thesecond light, and the frequencies of the respective sideband lights inthe order d of −2 to +3.

In FIG. 11 , f₂−(f₁+2fm) is a differential frequency between thefrequency of f₂ included in the second light and the frequency f₁+2fm ofthe order of +2 sideband light.

In FIG. 11 , only the offset frequency of which the differentialfrequency is f₂−(f₁+2fm) is illustrated. However, actually, in additionto the offset frequency of which the differential frequency isf₂−(f₁+2fm), an offset frequency of which the differential frequency isf₂−(f₁+fm), an offset frequency of which the differential frequency isf₂−(f₁+3fm), or the like may be included in the multiplexed light.

When the phase difference detected by the phase comparator 53 of the PLLcircuit 22 converges and phase synchronization of the PLL circuit 22 isfulfilled, the following Expression (5) is established.

$\begin{matrix}{{f_{2} - f_{1}} = {{\frac{N}{R}{fr}} + {d \times {fm}}}} & (5)\end{matrix}$

Therefore, the frequency difference between the first frequency f₁included in the first synchronization light included in the offsetlocking light and the second frequency f₂ included in the secondsynchronization light included in the offset locking light converges inaccordance with Expression (5).

In the third embodiment described above, the optical frequency controldevice 1 is configured in such a manner that the signal processing unit81 determines the order d of the sideband light on the basis of the setvalue Δf_(set) and sets the first frequency division number N inaccordance with each of the set value Δf_(set) and the order d.Therefore, similarly to the first embodiment, the optical frequencycontrol device 1 can change the frequency difference between the firstfrequency included in the first light and the second frequency includedin the second light without using a frequency synthesizer.

Moreover, in the optical frequency control device 1, when the signalprocessing unit 81 determines the order d of the sideband light to be ahigh order, it is possible to obtain the offset locking light having ahigh frequency of the order d.

Even when the signal processing unit 81 determines the order d of thesideband light to be the high order, the response speed in each of thephotodiode 19 and the PLL circuit 22 is the same as the response speedin a case where the order d of the sideband light is low. Therefore,even if the order d is changed, it is unnecessary to replace each of thephotodiode 19 and the PLL circuit 22.

Fourth Embodiment

In a fourth embodiment, an optical frequency control device 1, in whicha signal processing unit 100 switches the polarity of a phase errorsignal output from a light source control circuit 90, will be described.

FIG. 12 is a configuration diagram illustrating an optical oscillationdevice 2 including the optical frequency control device 1 according tothe fourth embodiment. In FIG. 12 , the same reference signs as those inFIGS. 1 and 8 denote the same or corresponding parts, and thus thedescription thereof is omitted.

In the optical frequency control device 1 illustrated in FIG. 12 , alight source control circuit 90 and the signal processing unit 100 areapplied to the optical frequency control device 1 illustrated in FIG. 8. However, this is merely an example, and the light source controlcircuit 90 and the signal processing unit 100 may be applied to theoptical frequency control device 1 illustrated in FIG. 1 .

The light source control circuit 90 includes a PLL circuit 91 and theloop filter 23.

Similarly to the light source control circuit 21 illustrated in FIGS. 1and 8 , the light source control circuit 90 frequency-divides adifferential beat signal detected by the detection circuit 16 by a firstfrequency division number N and frequency-divides a reference signaloscillated by a reference signal source 20 by a second frequencydivision number R.

Unlike the light source control circuits 21 illustrated in FIGS. 1 and 8, the light source control circuit 90 sets the polarity of the phaseerror signal indicating the phase difference between thefrequency-divided differential beat signal and the frequency-dividedreference signal in accordance with the polarity set value output fromthe signal processing unit 100.

The light source control circuit 90 outputs a phase error signal havingthe set polarity to the second light source 12.

The PLL circuit 91 frequency-divides the differential beat signal outputfrom the photodiode 19 by the first frequency division number N.

The PLL circuit 91 frequency-divides the reference signal output fromthe reference signal source 20 by the second frequency division numberR.

The PLL circuit 91 sets the polarity of the phase error signalindicating the phase difference between the frequency-divideddifferential beat signal and the frequency-divided reference signal inaccordance with the polarity set value output from the signal processingunit 100.

The PLL circuit 91 outputs the phase error signal having the setpolarity to the loop filter 23.

Similarly to the signal processing unit 81 illustrated in FIG. 10 , thesignal processing unit 100 determines the order d of the sideband lighton the basis of the set value Δf_(set) and sets the first frequencydivision number N according to each of the set value Δf_(set) and theorder d.

The signal processing unit 100 outputs each of the first frequencydivision number N and the second frequency division number R to the PLLcircuit 91.

Unlike the signal processing unit 81 illustrated in FIG. 10 , the signalprocessing unit 100 outputs a polarity set value indicating the polarityof the phase error signal to the PLL circuit 91.

Similarly to the signal processing unit 81 illustrated in FIG. 10 , thesignal processing unit 100 sets each of the first control signal forcontrolling the first frequency f₁ and the second control signal forcontrolling the second frequency f₂ in accordance with the set valueΔf_(set).

The signal processing unit 100 outputs the first control signal to afirst light source 11 and outputs the second control signal to a secondlight source 12.

Similarly to the signal processing unit 81 illustrated in FIG. 10 , thesignal processing unit 100 adjusts each of the first control signal andthe second control signal on the basis of the voltage data output fromthe voltage monitor 71.

The signal processing unit 100 adjusts each of the first control signaland the second control signal on the basis of the voltage data. However,this is merely an example, and each of the first control signal and thesecond control signal may be not adjusted on the basis of the voltagedata, similarly to the signal processing unit 24 illustrated in FIG. 1 .

FIG. 13 is a configuration diagram illustrating the PLL circuit 91. InFIG. 13 , the same reference signs as those in FIG. 4 denote the same orcorresponding parts, and thus the description thereof is omitted.

If the polarity set value output from the signal processing unit 100indicates that the polarity of the phase error signal is set to bepositive, a polarity switch 54 sets the polarity of the phase errorsignal output from the phase comparator 53 to be positive.

If the polarity set value output from the signal processing unit 100indicates that the polarity of the phase error signal is set to benegative, the polarity switch 54 sets the polarity of the phase errorsignal output from the phase comparator 53 to be negative.

FIG. 14 is a configuration diagram illustrating the signal processingunit 100. In FIG. 14 , the same reference signs as those in FIGS. 5, 9,and 10 denote the same or corresponding parts, and thus the descriptionthereof is omitted.

The signal processing unit 100 illustrated in FIG. 14 includes a voltagerange storing unit 73, the comparator 74 and the control signal settingunit 75, similarly to the signal processing unit 72 illustrated in FIG.9 and the signal processing unit 81 illustrated in FIG. 10 . However,this is merely an example, and the signal processing unit 81 may includethe control signal setting unit 62 illustrated in FIG. 5 instead of thevoltage range storing unit 73, the comparator 74 and the control signalsetting unit 75.

The frequency division number setting unit 101 includes a subtractor 101a, a comparator 101 b, an increment unit 101 c, a multiplier 101 d, anabsolute value calculator 101 e, a divider 101 f, a multiplier 101 g,and a sign extractor 101 h.

The internal memory of the frequency division number setting unit 101stores each of the modulation frequency fm, the frequency fr and thesecond frequency division number R.

In the signal processing unit 100 illustrated in FIG. 14 , each of themodulation frequency fm, the frequency fr, and the second frequencydivision number R is stored in the internal memory of the frequencydivision number setting unit 101. However, this is merely an example,and each of the modulation frequency fm, the frequency fr, and thesecond frequency division number R may be given from the outside of thesignal processing unit 100.

For example, when a set value Δf_(set) of the frequency difference Δf isgiven from the outside of the device, the frequency division numbersetting unit 101 determines the order d of the sideband light based oneach of the set value Δf_(set) and the modulation frequency fm.

The frequency division number setting unit 101 sets the first frequencydivision number N in accordance with each of the set value Δf_(set), theorder d, the modulation frequency fm, the frequency fr and the secondfrequency division number R.

The frequency division number setting unit 101 outputs each of the firstfrequency division number N and the second frequency division number Rto the PLL circuit 91.

The frequency division number setting unit 101 sets the polarity of thephase error signal in accordance with each of the set value Δf_(set),the order d, and the modulation frequency fm and outputs the polarityset value indicating the polarity to the PLL circuit 91.

For example, when the set value Δf_(set) of the frequency difference Δfis given from the outside of the device, the subtractor 101 a subtractsthe multiplication result d×fm of the multiplier 101 d from the setvalue Δf_(set) and outputs the subtraction result Δf_(set)−d×fm to thecomparator 101 b.

The comparator 101 b compares the subtraction result Δf_(set)−d×fmoutput from the subtractor 101 a with −fm/2, and also compares thesubtraction result Δf_(set)−d×fm with +fm/2.

When the subtraction result Δf_(set)−d×fm is equal to or less than −fm/2or the subtraction result Δf_(set)−d×fm is equal to or greater than+fm/2, the comparator 101 b outputs an increment command of the order dto the increment unit 101 c.

When the subtraction result Δf_(set)−d×fm is greater than −fm/2 and lessthan +fm/2, the comparator 101 b determines the order d at the presenttime as the order d of the sideband light, and outputs the subtractionresult Δf_(set)−d×fm to each of an absolute value calculator 101 e and asign extractor 101 h.

When receiving the increment command from the comparator 101 b, theincrement unit 101 c increments the order d by 1. The initial value ofthe order d is 1.

The multiplier 101 d multiplies the order d output from the incrementunit 101 c by the modulation frequency fm, and outputs a multiplicationresult d×fm of the order d by the modulation frequency fm to thesubtractor 101 a.

The absolute value calculator 101 e calculates the absolute value|Δf_(set)−d×fm| of the subtraction result Δf_(set)−d×fm output from thecomparator 101 b, and outputs the absolute value |Δf_(set)−d×fm| to themultiplier 101 g.

The divider 101 f divides the second frequency division number R by thefrequency fr and outputs a division result R/fr to the multiplier 101 g.

The multiplier 101 g multiplies the absolute value |Δf_(set)−d×fm|output from the absolute value calculator 101 e by the division resultR/fr output from the divider 101 f, and outputs the multiplicationresult |Δf_(set)−d×fm|×R/fr as the first frequency division number N tothe prescaler 51 of the PLL circuit 91.

For example, when the second frequency f₂ is greater than the firstfrequency f₁, if the subtraction result Δf_(set)−d×fm output from thecomparator 101 b is equal to or greater than 0, a sign extractor 101 houtputs a polarity set value indicating a positive polarity to thepolarity switch 54 of the PLL circuit 91.

For example, when the second frequency f₂ is greater than the firstfrequency f₁, if the subtraction result Δf_(set)−d×fm output from thecomparator 101 b is less than 0, the sign extractor 101 h outputs apolarity set value indicating the negative polarity to the polarityswitch 54.

For example, when the first frequency f₁ is equal to or greater than thesecond frequency f₂, if the subtraction result Δf_(set)−d×fm output fromthe comparator 101 b is equal to or greater than 0, the sign extractor101 h outputs a polarity set value indicating the negative polarity tothe polarity switch 54.

For example, when the first frequency f₁ is equal to or greater than thesecond frequency f₂, if the subtraction result Δf_(set)−d×fm output fromthe comparator 101 b is less than 0, the sign extractor 101 h outputsthe polarity set value indicating the positive polarity to the polarityswitch 54.

Next, the operation of the optical frequency control device 1illustrated in FIG. 12 will be described. Note that, since the opticalfrequency control device 1 is similar to the optical frequency controldevice 1 illustrated in FIG. 8 except for the PLL circuit 91 of thelight source control circuit 90 and the frequency division numbersetting unit 101 of the signal processing unit 100, only the operationsof the PLL circuit 91 and the frequency division number setting unit 101will be described herein.

For example, when a set value Δf_(set) of the frequency difference Δf isgiven from the outside of the device, the subtractor 101 a of thefrequency division number setting unit 101 subtracts the multiplicationresult d×fm of the multiplier 101 d from the set value Δf_(set). Notethat, since the initial value of the order d is 1, the multiplicationresult d×fm is equal to fm at the present time.

The subtractor 101 a outputs the subtraction result Δf_(set)−d×fm to thecomparator 101 b.

The comparator 101 b compares the subtraction result Δf_(set)−d×fmoutput from the subtractor 101 a with −fm/2, and also compares thesubtraction result Δf_(set) d×fm with +fm/2.

As shown in the following Expression (6), when the subtraction resultΔf_(set)−d×fm is equal to or less than −fm/2, or the subtraction resultΔf_(set)−d×fm is equal to or greater than +fm/2, the comparator 101 boutputs an increment command of the order d to the increment unit 101 c.Δf _(set) −d×fm≤−fm/2ORΔf _(set) −d×fm≥fm/2   (6)

When the subtraction result Δf_(set)−d×fm is greater than −fm/2 and lessthan +fm/2 as expressed in the following Expression (7), the comparator101 b determines the order d at the present time as the order d of thesideband light.−fm/2<Δf _(set) −d×fm<fm/2  (7)

When determining the order d of the sideband light, the comparator 101 boutputs the subtraction result Δf_(set)−d×fm to each of the absolutevalue calculator 101 e and the sign extractor 101 h.

Herein, for convenience of explanation, it is assumed that thesubtraction result Δf_(set)−d×fm is equal to or less than −fm/2 or less,or the subtraction result Δf_(set)−d×fm is equal to or greater than+fm/2 or more, and the comparator 101 b outputs an increment command ofthe order d to the increment unit 101 c.

When receiving the increment command from the comparator 101 b, theincrement unit 101 c increments the order d by 1. Since the order dbefore being incremented is 1, the order d output from the incrementunit 101 c is 2.

The multiplier 101 d multiplies the order d output from the incrementunit 101 c by the modulation frequency fm, and outputs a multiplicationresult d×fm of the order d by the modulation frequency fm to thesubtractor 101 a.

Upon receiving the multiplication result d×fm from the multiplier 101 d,the subtractor 101 a subtracts the multiplication result d×fm from theset value Δf_(set) of the frequency difference Δf. Since the order doutput from the increment unit 101 c is 2, the multiplication resultd×fm is equal to 2fm.

The subtractor 101 a outputs the subtraction result Δf_(set)−d×fm to thecomparator 101 b.

The comparator 101 b compares the subtraction result Δf_(set)−d×fmoutput from the subtractor 101 a with −fm/2, and also compares thesubtraction result Δf_(set) d×fm with +fm/2.

As shown in Expression (6), when the subtraction result Δf_(set)−d×fm isequal to or less than −fm/2, or the subtraction result Δf_(set)−d×fm isequal to or greater than +fm/2, the comparator 101 b outputs anincrement command of the order d to the increment unit 101 c.

When the subtraction result Δf_(set)−d×fm is greater than −fm/2 and lessthan +fm/2 as expressed in Expression (7), the comparator 101 bdetermines the order d at the present time as the order d of thesideband light.

The comparator 101 b outputs the subtraction result Δf_(set)−d×fm toeach of the absolute value calculator 101 e and the sign extractor 101h.

Herein, for convenience of description, it is assumed that thesubtraction result Δf_(set)−d×fm is greater than −fm/2 and less than+fm/2, and the comparator 101 b outputs the subtraction resultΔf_(set)−d×fm to each of the absolute value calculator 101 e and thesign extractor 101 h.

Upon receiving the subtraction result Δf_(set)−d×fm from the comparator101 b, the absolute value calculator 101 e calculates the absolute value|Δf_(set)−d×fm| of the subtraction result Δ_(set) d×fm.

The absolute value calculator 101 e outputs the absolute value|Δf_(set)−d×fm| to the multiplier 101 g.

The divider 101 f acquires the second frequency division number R storedin the internal memory and the frequency fr included in the referencesignal stored in the internal memory.

The divider 101 f divides the second frequency division number R by thefrequency fr and outputs a division result R/fr to the multiplier 101 g.

The multiplier 101 g multiplies the absolute value |Δf_(set)−d×fm|output from the absolute value calculator 101 e by the division resultR/fr output from the divider 101 f.

The multiplication result |Δf_(set)−d×fm|×R/fr of the absolute value|Δf_(set)−d×fm| and the division result R/fr corresponds to the firstfrequency division number N as shown in Expression (8).

The multiplier 101 g outputs the multiplication result|Δf_(set)−d×fm|×R/fr to the prescaler 51 of the PLL circuit 91 as thefirst frequency division number N.

$\begin{matrix}{N = {\frac{R}{fr}{{{\Delta\; f_{set}} - {d \times {fm}}}}}} & (8)\end{matrix}$

For example, when the second frequency f₂ is greater than the firstfrequency f₁, if the subtraction result Δf_(set)−d×fm output from thecomparator 101 b is equal to or greater than 0 as shown by the followingExpression (9), the sign extractor 101 h outputs a polarity set valueindicating a positive polarity to the polarity switch 54 of the PLLcircuit 91.Δf _(set) −d×fm≥0  (9)

For example, when the second frequency f₂ is greater than the firstfrequency f₁, if the subtraction result Δf_(set)−d×fm output from thecomparator 101 b is less than 0 as shown by the following Expression(10), the sign extractor 101 h outputs a polarity set value indicatingthe negative polarity to the polarity switch 54.Δf _(set) −d×fm<0  (10)

For example, when the first frequency f₁ is equal to or greater than thesecond frequency f₂, if the subtraction result Δf_(set)−d×fm output fromthe comparator 101 b is equal to or greater than 0 as shown byExpression (9), the sign extractor 101 h outputs a polarity set valueindicating the negative polarity to the polarity switch 54.

For example, when the first frequency f₁ is equal to or greater than thesecond frequency f₂, if the subtraction result Δf_(set)−d×fm output fromthe comparator 101 b is less than 0 as shown by Expression (10), thesign extractor 101 h outputs a polarity set value indicating thepositive polarity to the polarity switch 54.

The prescaler 51 of the PLL circuit 91 frequency-divides thedifferential beat signal output from the photodiode 19 by the firstfrequency division number N output from the multiplier 101 g of thefrequency division number setting unit 101.

Since the frequency included in the differential beat signal isf₂−(f₁+d×fm), the frequency of the differential beat signal after thefrequency division by the prescaler 51 is (f₂−(f₁+d×fm))/N.

The prescaler 51 outputs the divided differential beat signal to thepolarity switch 54.

The prescaler 52 of the PLL circuit 91 frequency-divides the referencesignal output from the reference signal source 20 by the secondfrequency division number R output from the frequency division numbersetting unit 101.

Since the frequency included in the reference signal is fr, thereference signal after the frequency division by the prescaler 52 isfr/R.

The prescaler 52 outputs the divided reference signal to the polarityswitch 54.

If the polarity set value output from the sign extractor 101 h indicatesthat the polarity of the phase error signal is set to be positive, thepolarity switch 54 of the PLL circuit 91 sets the polarity of the phaseerror signal output from the phase comparator 53 to be positive.

If the polarity set value output from the sign extractor 101 h indicatesthat the polarity of the phase error signal is set to be negative, thepolarity switch 54 sets the polarity of the phase error signal outputfrom the phase comparator 53 to be negative.

When the phase difference detected by the phase comparator 53 of the PLLcircuit 91 converges and phase synchronization of the PLL circuit 91 isfulfilled, the following Expression (11) is established.

$\begin{matrix}{{f_{2} - f_{1}} = {{\frac{N}{R}{fr}} + {d \times {fm}}}} & (11)\end{matrix}$

Therefore, the frequency difference between the first frequency f₁included in the first synchronization light included in the offsetlocking light and the second frequency f₂ included in the secondsynchronization light included in the offset locking light converges inaccordance with Expression (11).

FIG. 15 is an explanatory diagram illustrating the first frequency f₁included in the first light, the second frequency f₂ included in thesecond light, and the frequencies of the respective sideband light inthe order d of −2 to +2.

Herein, for convenience of description, it is assumed that the responsefrequency of the PLL circuit 91 is in a range of DC (direct current) toBpll. Bpll>0.

For example, it is assumed that the order d determined by the comparator101 b of the frequency division number setting unit 101 is 1 and thepolarity set by the polarity switch 54 of the frequency division numbersetting unit 101 is negative.

In this case, the second frequency f₂ included in the second light canbe converged in a range between the sideband frequency f₁+fm of the+first-order sideband light and a frequency lower by Bpll from thesideband frequency f₁+fm.

Next, it is assumed that the order d determined by the comparator 101 bis 1 and the polarity set by the polarity switch 54 is positive.

In this case, the second frequency f₂ included in the second light canbe converged in a range of the sideband frequency f₁+fm and a frequencyhigher by Bpll from the sideband frequency f₁+fm.

Therefore, if the order d is 1, the second frequency f₂ included in thesecond light can be changed by a range of 2Bpll. The range of 2Bpll istwice the variable width of the second frequency f₂ in a case where thepolarity of the phase error signal is fixed.

For example, it is assumed that the order d determined by the comparator101 b of the frequency division number setting unit 101 is 2 and thepolarity set by the polarity switch 54 of the frequency division numbersetting unit 101 is negative.

In this case, the second frequency f₂ included in the second light canbe converged to a range between the sideband frequency f₁+2fm includedin the +second-order sideband light and a frequency lower by Bpll fromthe sideband frequency f₁+2fm.

Next, it is assumed that the order d determined by the comparator 101 bis 2 and the polarity set by the polarity switch 54 is positive.

In this case, the second frequency f₂ included in the second light canbe converged in a range of the sideband frequency f₁+2fm and a frequencyhigher by Bpll from the sideband frequency f₁+2fm.

Therefore, if the order d is 2, the second frequency f₂ included in thesecond light can be changed by a range of 2Bpll. The range of 2Bpll istwice the variable width of the second frequency f₂ in a case where thepolarity of the phase error signal is fixed.

If the order d can be switched to n stages, the second frequency f₂included in the second light can be changed by a range of 2×n×Bpll.

In the fourth embodiment described above, the optical frequency controldevice 1 illustrated in FIG. 12 is configured in such a manner that thesignal processing unit 100 switches the polarity of the phase errorsignal output from the light source control circuit 90. Therefore,similarly to the optical frequency control device 1 illustrated in FIG.1 , the optical frequency control device 1 illustrated in FIG. 12 canchange the frequency difference between the first frequency included inthe first light and the second frequency included in the second lightwithout using the frequency synthesizer.

Moreover, the optical frequency control device 1 illustrated in FIG. 12can double the variable width of the second frequency included in thesecond light as compared with the optical frequency control device 1illustrated in FIG. 1 . Therefore, the response band of the PLL circuit91 can be relaxed to ½ of the response band of the PLL circuit 22illustrated in FIG. 1 .

Furthermore, as compared with the optical frequency control device 1illustrated in FIG. 1 , the optical frequency control device 1illustrated in FIG. 12 can reduce the possibility that the spuriouscaused by the mixing of the sideband of the desired order and thesideband of the order adjacent to the sideband is mixed in the PLLcircuit 91.

Fifth Embodiment

In a fifth embodiment, an optical frequency control device 1 including apower regulator 110 that regulates power of a local oscillation signaloscillated by a first local oscillation signal source 15 will bedescribed.

FIG. 16 is a configuration diagram illustrating an optical oscillationdevice 2 including the optical frequency control device 1 according tothe fifth embodiment. In FIG. 16 , the same reference signs as those inFIGS. 1, 8, and 12 denote the same or corresponding parts, and thus thedescription thereof is omitted.

The power regulator 110 is implemented, for example, by a variableattenuator.

The power regulator 110 regulates the power of the local oscillationsignal so that the power of the local oscillation signal oscillated bythe first local oscillation signal source 15 becomes modulation powerP_(m) output from a signal processing unit 120 described later, andoutputs the local oscillation signal after the power regulation to theLN phase modulator 17 of the detection circuit 16.

The signal processing unit 100 performs the operation similar to that ofthe signal processing unit 120 illustrated in FIG. 12 , and controls theregulation of power by the power regulator 110 on the basis of the orderd determined by the comparator 101 b.

In the optical frequency control device 1 illustrated in FIG. 16 , thesignal processing unit 120 performs the operation similar to that of thesignal processing unit 100 illustrated in FIG. 12 . However, this ismerely an example, and the signal processing unit 120 may perform theoperation similar to that of the signal processing unit 81 illustratedin FIG. 10 and control the regulation of the power by the powerregulator 110 on the basis of the order d determined by the comparator82 b.

FIG. 17 is a configuration diagram illustrating the signal processingunit 120. In FIG. 17 , the same reference signs as those in FIG. 14denote the same or corresponding parts, and thus description thereof isomitted.

A power calculator 121 calculates the modulation power P_(m) of thepower regulator 110 based on the order d determined by the comparator101 b, and outputs the modulation power P_(m) to the power regulator110.

Next, the operation of the optical frequency control device 1illustrated in FIG. 16 will be described. Note that, since the opticalfrequency control device 1 is similar to the optical frequency controldevice 1 illustrated in FIG. 12 except for the power regulator 110 andthe power calculator 121 of the signal processing unit 120, only theoperations of the power regulator 110 and the power calculator 121 willbe described herein.

The optical frequency control device 1 illustrated in FIG. 16 uses an LNphase modulator 17 as a sideband generator.

In a case where the optical frequency control device 1 uses the LN phasemodulator 17, the optical power of the sideband light of the order d isproportional to the square of the Bessel function of the first type(|J_(d)(m)|₂) m is a modulation index by the LN phase modulator 17 andis expressed by the following Expression (12).

$\begin{matrix}{{m\lbrack{rad}\rbrack} = {\pi\frac{\sqrt{2{ZP}_{m}}}{V_{\pi}}}} & (12)\end{matrix}$

In Expression (12), V_(π) is the half wave voltage [V] of the LN phasemodulator 17, Z is the input impedance [ohm] of the modulation port inthe LN phase modulator 17, and P_(m) is the modulation power [W].

FIG. 18 is an explanatory diagram illustrating a calculation example ofa sideband level with respect to modulation power of the LN phasemodulator 17. In the calculation example of FIG. 18 , the half wavevoltage V_(π) of the LN phase modulator 17 is 3 [V], and the inputimpedance Z of the modulation port in the LN phase modulator 17 is 50[ohm].

In FIG. 18 , the horizontal axis represents the modulation power P_(m)[W], and the vertical axis represents the sideband level [dB].

“First-order lower MIX” means that the second light is mixed on the lowfrequency side of the primary sideband light, and “first-order upperMIX” means that the second light is mixed on the high frequency side ofthe first-order sideband light. “Second-order lower MIX” means that thesecond light is mixed on the low frequency side of the secondarysideband light, and “second-order upper MIX” means that the second-orderlight is mixed on the high frequency side of the secondary sidebandlight.

For example, in a case where the second frequency f₂ included in thesecond light is generated on the low frequency side of the sidebandlight having the order d of the second order, the adjacent sidebandlight that becomes a spurious factor is the sideband light having theorder d of the first order.

As illustrated in FIG. 18 , when the modulation power P_(m) is about 135[mW], the first-order sideband light is suppressed, which enables alevel ratio of the second-order sideband light to the first-ordersideband light to be high, so that the optical oscillation device 2 cansuppress main spurious at the time of detecting the differential beatsignal.

A power calculator 121 calculates the modulation power P_(m) of thepower regulator 110 based on the order d determined by the comparator101 b, and outputs the modulation power P_(m) to the power regulator110.

That is, the power calculator 121 calculates the modulation power P_(m)that maximizes the level ratio of the sideband light of the order d withrespect to the adjacent sideband light of the sideband light of theorder d determined by the comparator 101 b.

The modulation power P_(m) calculated by the power calculator 121 is notlimited to modulation power that maximizes the level ratio of thesideband light of the order d to the adjacent sideband light, and may bemodulation power that makes the level of the sideband light of the orderd greater than the level of the adjacent sideband light.

The power calculator 121 outputs the calculated modulation power P_(m)to the power regulator 110.

The power regulator 110 regulates the power of the local oscillationsignal so that the power of the local oscillation signal oscillated bythe first local oscillation signal source 15 becomes the modulationpower P_(m) output from the power calculator 121.

The power regulator 110 outputs the local oscillation signal after thepower regulation to the LN phase modulator 17.

In the fifth embodiment described above, the optical frequency controldevice 1 illustrated in FIG. 16 is configured to include the powerregulator 110 that regulates the power of the local oscillation signaloscillated by the first local oscillation signal source 15, and thesignal processing unit 120 controls the regulation of the power by thepower regulator 110 on the basis of the order of the sideband light.Therefore, similarly to the optical frequency control device 1illustrated in FIG. 1 , the optical frequency control device 1illustrated in FIG. 16 can change the frequency difference between thefirst frequency included in the first light and the second frequencyincluded in the second light without using the frequency synthesizer.

Moreover, the optical frequency control device 1 illustrated in FIG. 16can reduce the possibility that spurious is mixed into the PLL circuit91 as compared with the optical frequency control device 1 illustratedin FIG. 1 .

In addition, the optical frequency control device 1 illustrated in FIG.16 can avoid interference between two signals occurring under thecondition of f₂−f₁=(n+0.5)×fm where d is an integer. That is,interference between the first-order differential beat signal and thesecond-order differential beat signal can be avoided. Therefore, evenunder the condition that the band Bpll of the PLL circuit 91 is ½ of themodulation frequency fm, the PLL circuit 91 can be prevented from beingout of synchronization.

Sixth Embodiment

In a sixth embodiment, an optical frequency control device 1, in which adetection circuit 152 detects a differential beat signal including adifferential frequency between a frequency of sideband light included inmodulated first light and a frequency of sideband light included inmodulated second light, will be described.

FIG. 19 is a configuration diagram illustrating an optical oscillationdevice 2 including the optical frequency control device 1 according tothe sixth embodiment. In FIG. 19 , the same reference signs as those inFIGS. 1, 8, 12, and 16 denote the same or corresponding parts, and thusthe description thereof is omitted.

A second local oscillation signal source 151 oscillates a localoscillation signal whose frequency is fn.

The second local oscillation signal source 151 is connected to an LNphase modulator 153 of a detection circuit 152 via, for example, anoptical fiber, and outputs the local oscillation signal to the LN phasemodulator 153.

The detection circuit 152 includes an LN phase modulator 17, the LNphase modulator 153, an optical multiplexer 154, and a photodiode 155.

The detection circuit 152 receives first light from a first light source11 and receives second light from a second light source 12.

The detection circuit 152 modulates the first light oscillated by thefirst light source 11 by the local oscillation signal oscillated by thefirst local oscillation signal source 15, and modulates the second lightoscillated by the second light source 12 by the local oscillation signaloscillated by the second local oscillation signal source 151.

The detection circuit 152 detects a differential beat signal including adifferential frequency (f₂−fn)−(f₁+fm) between the frequency f₁+fm ofthe sideband light included in the modulated first light and thefrequency f₂−fn of the sideband light included in the modulated secondlight.

The detection circuit 152 outputs the differential beat signal to thelight source control circuit 21.

The LN phase modulator 153 modulates the second split light output froma second optical distributor 14 by the local oscillation signaloscillated by the second local oscillation signal source 151, therebygenerating sideband light having a frequency of f₂±fn. fn is amodulation frequency by the LN phase modulator 153.

The LN phase modulator 153 is connected to the optical multiplexer 154via, for example, an optical fiber, and outputs modulation lightincluding each of the second frequency f₂ and the frequency f₂±fn to theoptical multiplexer 154 as the modulated second light.

In the optical frequency control device 1 illustrated in FIG. 19 , thedetection circuit 152 includes an LN phase modulator 153. However, theembodiment is not limited to the LN phase modulator 153 as long as themodulator can generate sideband light, and the detection circuit 152 mayinclude a Mach-Zehnder intensity modulator, an electro-absorptionmodulator, or the like.

The optical multiplexer 154 multiplexes the modulation light output fromthe LN phase modulator 17 and the modulation light output from the LNphase modulator 153.

The optical multiplexer 154 is connected to the photodiode 155 via, forexample, an optical fiber, and outputs multiplexed light of themodulation light output from the LN phase modulator 17 and themodulation light output from the LN phase modulator 153 to thephotodiode 155.

The photodiode 155 converts the multiplexed light output from theoptical multiplexer 154 into an electric signal.

The photodiode 155 detects a differential frequency (f₂−fn)−(f₁+fm)between the frequency f₁+fm of the sideband light included in themodulated first light and the frequency f₂−fn of the sideband lightincluded in the modulated second light from the electric signal.

The photodiode 155 outputs a signal including a differential frequency(f₂−fn)−(f₁+fm) as a differential beat signal to the PLL circuit 22 ofthe light source control circuit 21.

In the optical oscillation device 2 illustrated in FIG. 19 , the secondlocal oscillation signal source 151 and the detection circuit 152 areapplied to the optical oscillation device 2 illustrated in FIG. 1 .However, this is merely an example, and the second local oscillationsignal source 151 and the detection circuit 152 may be applied to theoptical oscillation device 2 illustrated in FIG. 8 , the opticaloscillation device 2 illustrated in FIG. 12 , or the optical oscillationdevice 2 illustrated in FIG. 16 .

Next, the operation of the optical oscillation device 2 illustrated inFIG. 19 will be described.

The first local oscillation signal source 15 oscillates a localoscillation signal having a frequency of fm and outputs the localoscillation signal to the LN phase modulator 17.

The LN phase modulator 17 modulates the first split light output fromthe first optical distributor 13 by the local oscillation signaloscillated by the first local oscillation signal source 15, therebygenerating sideband light (see FIG. 20 ) having a frequency of f₁±fm.The phase of the modulation frequency fm by the LN phase modulator 17 isstable, for example, in the microwave region.

The LN phase modulator 17 outputs the modulation light including each ofthe first frequency f₁ and the frequency f₁±fm to the opticalmultiplexer 154.

The second local oscillation signal source 151 oscillates a localoscillation signal having a frequency of fn and outputs the localoscillation signal to the LN phase modulator 153.

The LN phase modulator 153 modulates the second split light output fromthe second optical distributor 14 by the local oscillation signaloscillated by the second local oscillation signal source 151, therebygenerating sideband light (see FIG. 20 ) having a frequency of f₂±fn.The phase of the modulation frequency fn by the LN phase modulator 153is stable, for example, in the microwave region.

The LN phase modulator 153 outputs the modulation light including eachof the second frequency f₂ and the frequency f₂±fn to the opticalmultiplexer 154.

FIG. 20 is an explanatory diagram illustrating a first frequency f₁included in first light, frequencies f₁−fm and f₁+fm of sideband lightincluded in modulated first light, a second frequency f₂ included insecond light, and frequencies f₂−fn and f₂+fn of sideband light includedin modulated second light.

In FIG. 20 , (f₂−fn)−(f₁+fm) is a differential frequency between thefrequency f₁+fm of the sideband light included in the modulated firstlight and the frequency f₂−fn of the sideband light included in themodulated second light.

In the optical frequency control device 1 illustrated in FIG. 19 , theLN phase modulator 17 generates sideband light having a frequency off₁±fm at an interval of the modulation frequency fm on both sides of thefirst frequency f₁ with the first frequency f₁ as the center. However,this is merely an example, and the LN phase modulator 17 may generate,for example, only sideband light having a frequency of f₁+fm on one sideof the first frequency f₁.

In the optical frequency control device 1 illustrated in FIG. 19 , theLN phase modulator 153 generates sideband light having a frequency off₂±fn at an interval of the modulation frequency fn on both sides of thesecond frequency f₂ with the second frequency f₂ as the center. However,this is merely an example, and the LN phase modulator 153 may generate,for example, only the sideband light having the frequency f₂−fn on oneside of the second frequency f₂.

The optical multiplexer 154 multiplexes the modulation light output fromthe LN phase modulator 17 and the modulation light output from the LNphase modulator 153.

The optical multiplexer 154 outputs the multiplexed light of themodulation light output from the LN phase modulator 17 and themodulation light output from the LN phase modulator 153 to thephotodiode 155.

As illustrated in FIG. 20 , the multiplexed light includes a firstfrequency f₁ included in the first light and frequencies f₁−fm and f₁+fmof the sideband light included in the modulated first light.

Moreover, as illustrated in FIG. 20 , the multiplexed light includes asecond frequency f₂ included in the second light and frequencies f₂−fnand f₂+fn of the sideband light included in the modulated second light.

When receiving the multiplexed light from the optical multiplexer 154,the photodiode 155 converts the multiplexed light into an electricsignal.

The photodiode 155 detects a differential frequency (f₂−fn)−(f₁+fm)between the frequency f₁+fm of the sideband light included in themodulated first light and the frequency f₂−fn of the sideband lightincluded in the modulated second light from the electric signal.

The photodiode 155 outputs a signal including the differential frequency(f₂−fn)−(f₁+fm) as a differential beat signal to the PLL circuit 22 ofthe light source control circuit 21.

In the optical frequency control device 1 illustrated in FIG. 19 ,assuming that the first frequency f₁<the second frequency f₂, thephotodiode 155 detects, from the electric signal, a differentialfrequency (f₂−fn)−(f₁+fm) between the frequency f₁+fm of the sidebandlight included in the modulated first light and the frequency f₂−fn ofthe sideband light included in the modulated second light. However, thisis merely an example, and for example, if the frequency f₁>the frequencyf₂, the photodiode 155 may detect, from the electric signal, thedifferential frequency (f₁−fm)−(f₂+fn) between the frequency f₁−fm ofthe sideband light included in the modulated first light and thefrequency f₂+fn of the sideband light included in the modulated secondlight. In this case, the photodiode 155 outputs a signal including adifferential frequency (f₁−fn)−(f₂+fn) to the PLL circuit 22 as adifferential beat signal.

The reference signal source 20 oscillates a reference signal having afrequency fr and outputs the reference signal to the PLL circuit 22 ofthe light source control circuit 21.

The light source control circuit 21 frequency-divides the differentialbeat signal output from the photodiode 155 by a first frequency divisionnumber N and frequency-divides the reference signal output from thereference signal source 20 by a second frequency division number R.

The light source control circuit 21 outputs a phase error signalindicating a phase difference between the frequency-divided differentialbeat signal and the frequency-divided reference signal to the secondlight source 12, thereby changing the second frequency f₂ included inthe second light oscillated by the second light source 12.

Hereinafter, the operation of the light source control circuit 21 willbe described in detail.

The prescaler 51 of the PLL circuit 22 frequency-divides thedifferential beat signal output from the photodiode 155 by the firstfrequency division number N output from the signal processing unit 24.

Since the frequency included in the differential beat signal is(f₂−fn)−(f₁+fm), the frequency of the differential beat signal after thefrequency division by the prescaler 51 is ((f₂−fn)−(f₁+fm))/N.

The prescaler 51 outputs the frequency-divided differential beat signalto the phase comparator 53.

The prescaler 52 of the PLL circuit 22 frequency-divides the referencesignal output from the reference signal source 20 by the secondfrequency division number R output from the signal processing unit 24.

Since the frequency included in the reference signal is fr, thereference signal after the frequency division by the prescaler 52 isfr/R.

The prescaler 52 outputs the frequency-divided reference signal to thephase comparator 53.

The phase comparator 53 of the PLL circuit 22 detects a phase differencebetween the frequency-divided differential beat signal output from theprescaler 51 and the frequency-divided reference signal output from theprescaler 52.

The phase comparator 53 outputs a phase error signal indicating a phasedifference to the loop filter 23.

The loop filter 23 integrates the phase error signal output from thephase comparator 53 and outputs the integrated phase error signal to thesecond light source 12.

When the phase error signal after the integration is output to thesecond light source 12, the second frequency f₂ included in the secondlight output from the second light source 12 changes.

When the phase difference detected by the phase comparator 53 convergesand phase synchronization of the PLL circuit 22 is fulfilled, thefollowing Expression (13) is established.

$\begin{matrix}{\frac{\left( {\left( {f_{2} - f_{n}} \right) - \left( {f_{1} + {fm}} \right)} \right.}{N} = \frac{fr}{R}} & (13)\end{matrix}$

Expression (13) can be organized as the following Expression (14).

$\begin{matrix}{{f_{2} - f_{1}} = {{\frac{N}{R}{fr}} + {fm} + {fn}}} & (14)\end{matrix}$

Expression (14) represents an offset frequency f₂−f₁ between the firstfrequency f₁ included in the first light oscillated by the first lightsource 11 and the second frequency f₂ included in the second lightoscillated by the second light source 12. The offset frequency f₂−f₁ isdetermined by the first frequency division number N, the secondfrequency division number R, the frequency fr included in the referencesignal, the modulation frequency fm, and the modulation frequency fnincluded in the reference signal.

The offset frequency f₂−f₁ corresponds to a frequency difference betweenthe first frequency f₁ included in the first synchronization light andthe second frequency f₂ included in the second synchronization light.The first synchronization light and the second synchronization light arelight forming offset locking light.

When Expression (13) is solved for N, the following Expression (15) isobtained.

$\begin{matrix}\begin{matrix}{N = {\frac{R}{fr}\left( {f_{2} - f_{1} - {fm} - {fn}} \right)}} \\{= {\frac{R}{fr}\left( {{\Delta\; f_{set}} - {fm} - {fn}} \right)}}\end{matrix} & (15)\end{matrix}$

Upon receiving the set value Δf_(set) of the frequency difference Δfbetween the first frequency f₁ and the second frequency f₂ from theoutside of the device, the signal processing unit 24 sets each of thefirst frequency division number N and the second frequency divisionnumber R in accordance with the set value Δf_(set) as the firstembodiment.

The signal processing unit 24 outputs each of the first frequencydivision number N and the second frequency division number R to the PLLcircuit 22.

Similarly to the first embodiment, the signal processing unit 24 setseach of the first control signal for controlling the first frequency f₁and the second control signal for controlling the second frequency f₂ inaccordance with the set value Δf_(set).

The signal processing unit 24 outputs the first control signal to thefirst light source 11 and outputs the second control signal to secondlight source 12.

Similarly to the first embodiment, the first light source 11 changes thefirst frequency f₁ included in the first light in accordance with thefirst control signal output from the signal processing unit 24, andoscillates the first light including the changed first frequency f₁.

Thus, the first frequency f₁ included in the first light is adjusted inaccordance with the set value Δf_(set) of the frequency difference Δf.

The first light source 11 outputs the first light including the firstfrequency f₁ to the first optical distributor 13.

Similarly to the first embodiment, the second light source 12 changesthe second frequency f₂ included in the second light in accordance witheach of the phase error signal output from the loop filter 23 of thelight source control circuit 21 and the second control signal outputfrom the signal processing unit 24, and oscillates the second lightincluding the changed second frequency f₂.

Thus, the second frequency f₂ included in the second light is adjustedin accordance with the set value Δf_(set) of the frequency differenceΔf.

The second light source 12 outputs the second light including the secondfrequency f₂ to the second optical distributor 14.

When receiving the first light from the first light source 11, the firstoptical distributor 13 distributes the first light and outputs the firstsplit light, which is one light after the distribution, to the LN phasemodulator 17.

The first optical distributor 13 outputs the first synchronizationlight, which is the other light after the distribution, to the outsideof the device as one light of the offset locking light.

When receiving the second light from the second light source 12, thesecond optical distributor 14 distributes the second light and outputsthe second split light, which is one light after the distribution, tothe LN phase modulator 153.

The second optical distributor 14 outputs the second synchronizationlight, which is the other light after the distribution, to the outsideof the device as the other light in the offset locking light.

FIG. 21 is an explanatory diagram illustrating off set locking light.

In FIG. 21 , the frequency f₁ is the first frequency included in thefirst synchronization light, and the frequency f₂ is the secondfrequency included in the second synchronization light.

Since each of the first frequency f₁ and the second frequency f₂ isadjusted in accordance with the set value Δf_(set) of the frequencydifference Δf, the frequency difference Δf between the first frequencyf₁ and the second frequency f₂ can be changed.

In the sixth embodiment described above, the detection circuit 152modulates the second light oscillated by the second light source 12 bythe local oscillation signal oscillated by the second local oscillationsignal source 151. The optical frequency control device 1 illustrated inFIG. 19 is configured in such a manner that the detection circuit 152detects the differential beat signal including the differentialfrequency between the frequency of the sideband light included in themodulated first light and the frequency of the sideband light includedin the modulated second light instead of detecting the differential beatsignal including the differential frequency between the frequency of thesideband light included in the modulated first light and the secondfrequency. Therefore, similarly to the optical frequency control device1 illustrated in FIG. 1 , the optical frequency control device 1illustrated in FIG. 19 can change the frequency difference between thefirst frequency included in the first light and the second frequencyincluded in the second light without using a frequency synthesizer.

Seventh Embodiment

In a seventh embodiment, an optical frequency control device 1, in whicha detection circuit 156 modulates second light oscillated by a secondlight source 12 by a local oscillation signal oscillated by a firstlocal oscillation signal source 15, will be described.

FIG. 22 is a configuration diagram illustrating an optical oscillationdevice 2 including the optical frequency control device 1 according tothe seventh embodiment. In FIG. 22 , the same reference signs as thosein FIGS. 1, 8, 12, 16, and 19 denote the same or corresponding parts,and thus the description thereof is omitted.

The detection circuit 156 includes an LN phase modulator 17, an LN phasemodulator 157, an optical multiplexer 158, and a photodiode 159.

The detection circuit 156 receives the first light from the first lightsource 11 and receives the second light from the second light source 12.

The detection circuit 156 modulates the first light oscillated by thefirst light source 11 by the local oscillation signal oscillated by thefirst local oscillation signal source 15 and modulates the second lightoscillated by the second light source 12 by the local oscillation signaloscillated by the first local oscillation signal source 15.

The detection circuit 156 detects a differential beat signal including adifferential frequency (f₂−fm)−(f₁+fm) between the frequency f₁+fm ofthe sideband light included in the modulated first light and thefrequency f₂−fm of the sideband light included in the modulated secondlight.

The detection circuit 156 outputs the differential beat signal to thelight source control circuit 21.

The LN phase modulator 157 modulates the second split light output fromthe second optical distributor 14 by the local oscillation signaloscillated by the first local oscillation signal source 15, therebygenerating sideband light having a frequency of f₂±fm.

The LN phase modulator 157 is connected to the optical multiplexer 158via, for example, an optical fiber, and outputs modulation lightincluding each of the second frequency f₂ and the frequency f₂±fm to theoptical multiplexer 158 as the modulated second light.

In the optical frequency control device 1 illustrated in FIG. 22 , thedetection circuit 156 includes the LN phase modulator 157. However, itis not limited to the LN phase modulator 157 as long as the modulatorcan generate sideband light, and the detection circuit 156 may include aMach-Zehnder intensity modulator, an electro-absorption modulator, orthe like.

The optical multiplexer 158 multiplexes the modulation light output fromthe LN phase modulator 17 and the modulation light output from the LNphase modulator 157.

The optical multiplexer 158 is connected to the photodiode 159 via, forexample, an optical fiber, and outputs multiplexed light of themodulation light output from the LN phase modulator 17 and themodulation light output from the LN phase modulator 157 to thephotodiode 159.

The photodiode 159 converts the multiplexed light output from theoptical multiplexer 158 into an electric signal.

The photodiode 159 detects a differential frequency (f₂−fm)−(f₁+fm)between the frequency f₁+fm of the sideband light included in themodulated first light and the frequency f₂−fm of the sideband lightincluded in the modulated second light from the electric signal.

The photodiode 159 outputs a signal including the differential frequency(f₂−fm)−(f₁+fm) as a differential beat signal to the PLL circuit 22 ofthe light source control circuit 21.

In the optical oscillation device 2 illustrated in FIG. 22 , thedetection circuit 156 is applied to the optical oscillation device 2illustrated in FIG. 1 . However, this is merely an example, and thedetection circuit 156 may be applied to the optical oscillation device 2illustrated in FIG. 8 , the optical oscillation device 2 illustrated inFIG. 12 , or the optical oscillation device 2 illustrated in FIG. 16 .

Next, the operation of the optical oscillation device 2 illustrated inFIG. 22 will be described.

The first local oscillation signal source 15 oscillates a localoscillation signal having a frequency of fm and outputs the localoscillation signal to each of the LN phase modulator 17 and the LN phasemodulator 157.

The LN phase modulator 17 modulates the first split light output fromthe first optical distributor 13 by the local oscillation signaloscillated by the first local oscillation signal source 15, therebygenerating sideband light (see FIG. 23 ) having a frequency of f₁±fm.

The LN phase modulator 17 outputs the modulation light including each ofthe first frequency f₁ and the frequency f₁±fm to the opticalmultiplexer 158.

The LN phase modulator 157 modulates the second split light output fromthe second optical distributor 14 by the local oscillation signaloscillated by the first local oscillation signal source 15, therebygenerating sideband light (see FIG. 23 ) having a frequency of f₂±m.

The LN phase modulator 157 outputs the modulation light including eachof the second frequency f₂ and the frequency f₂±fm to the opticalmultiplexer 158.

FIG. 23 is an explanatory diagram illustrating a first frequency f₁included in first light, frequencies f₁−fm and f₁+fm of sideband lightincluded in modulated first light, a second frequency f₂ included insecond light, and frequencies f₂−fm and f₂+fm of sideband light includedin modulated second light.

In FIG. 23 , (f₂−fn)−(f₁+fm)=(f₂−f₁)−2fm is a differential frequencybetween the frequency f₁+fm of the sideband light included in themodulated first light and the frequency f₂−fm of the sideband lightincluded in the modulated second light.

In the optical frequency control device 1 illustrated in FIG. 22 , theLN phase modulator 17 generates sideband light having a frequency off₁+fm at an interval of the modulation frequency fm on both sides of thefirst frequency f₁ with the first frequency f₁ as the center. However,this is merely an example, and the LN phase modulator 17 may generate,for example, only sideband light having a frequency of f₁+fm on one sideof the first frequency f₁.

In the optical frequency control device 1 illustrated in FIG. 22 , theLN phase modulator 157 generates sideband light having a frequency off₂±fm at an interval of the modulation frequency fm on both sides of thesecond frequency f₂ with the second frequency f₂ as the center. However,this is merely an example, and the LN phase modulator 157 may generate,for example, only sideband light having a frequency of f₂+fm on one sideof the second frequency f₂.

The optical multiplexer 158 multiplexes the modulation light output fromthe LN phase modulator 17 and the modulation light output from the LNphase modulator 157.

The optical multiplexer 158 outputs the multiplexed light of themodulation light output from the LN phase modulator 17 and themodulation light output from the LN phase modulator 157 to thephotodiode 159.

As illustrated in FIG. 23 , the multiplexed light includes a firstfrequency f₁ included in the first light and frequencies f₁−fm and f₁+fmof the sideband light included in the modulated first light.

As illustrated in FIG. 23 , the multiplexed light includes a secondfrequency f₂ included in the second light and frequencies f₂−fm andf₂+fm of the sideband light included in the modulated second light.

When receiving the multiplexed light from the optical multiplexer 158,the photodiode 159 converts the multiplexed light into an electricsignal.

The photodiode 159 detects a differential frequency (f₂−f₁)−2fm betweenthe frequency f₁+fm of the sideband light included in the modulatedfirst light and the frequency f₂−fm of the sideband light included inthe modulated second light from the electric signal.

The photodiode 159 outputs a signal including the differential frequency(f₂−f₁)−2fm to the PLL circuit 22 of the light source control circuit 21as a differential beat signal.

In the optical frequency control device 1 illustrated in FIG. 22 ,assuming that the first frequency f₁<the second frequency f₂, thephotodiode 159 detects, from the electric signal, a differentialfrequency (f₂−f₁)−2fm between the frequency f₁+fm of the sideband lightincluded in the modulated first light and the frequency f₂−fm of thesideband light included in the modulated second light. However, this ismerely an example, and for example, if the frequency f₁>the frequencyf₂, the photodiode 159 may detect, from the electric signal, thedifferential frequency (f₁−f₂)−2fm between the frequency f₁−fm of thesideband light included in the modulated first light and the frequencyf₂+fm of the sideband light included in the modulated second light. Inthis case, the photodiode 159 outputs a signal including thedifferential frequency (f₁−f₂)−2fm to the PLL circuit 22 as adifferential beat signal.

The reference signal source 20 oscillates a reference signal having afrequency fr and outputs the reference signal to the PLL circuit 22 ofthe light source control circuit 21.

The light source control circuit 21 frequency-divides the differentialbeat signal output from the photodiode 155 by a first frequency divisionnumber N and frequency-divides the reference signal output from thereference signal source 20 by a second frequency division number R.

The light source control circuit 21 outputs a phase error signalindicating a phase difference between the frequency-divided differentialbeat signal and the frequency-divided reference signal to the secondlight source 12, thereby changing the second frequency f₂ included inthe second light oscillated by the second light source 12.

Hereinafter, the operation of the light source control circuit 21 willbe described in detail.

The prescaler 51 of the PLL circuit 22 frequency-divides thedifferential beat signal output from the photodiode 159 by the firstfrequency division number N output from the signal processing unit 24.

Since the frequency included in the differential beat signal is(f₂−f₁)−2fm, the frequency of the differential beat signal after thefrequency division by the prescaler 51 is (f₂−f₁)−2fm))/N.

The prescaler 51 outputs the frequency-divided differential beat signalto the phase comparator 53.

The prescaler 52 of the PLL circuit 22 frequency-divides the referencesignal output from the reference signal source 20 by the secondfrequency division number R output from the signal processing unit 24.

Since the frequency included in the reference signal is fr, thereference signal after the frequency division by the prescaler 52 isfr/R.

The prescaler 52 outputs the frequency-divided reference signal to thephase comparator 53.

The phase comparator 53 of the PLL circuit 22 detects a phase differencebetween the frequency-divided differential beat signal output from theprescaler 51 and the frequency-divided reference signal output from theprescaler 52.

The phase comparator 53 outputs a phase error signal indicating a phasedifference to the loop filter 23.

The loop filter 23 integrates the phase error signal output from thephase comparator 53 and outputs the integrated phase error signal to thesecond light source 12.

When the phase error signal after the integration is output to thesecond light source 12, the second frequency f₂ included in the secondlight output from the second light source 12 changes.

When the phase difference detected by the phase comparator 53 convergesand phase synchronization of the PLL circuit 22 is fulfilled, thefollowing Expression (16) is established.

$\begin{matrix}{\frac{\left( {\left( {f_{2} - f_{1}} \right) - {2{fm}}} \right)}{N} = \frac{fr}{R}} & (16)\end{matrix}$

Expression (16) can be organized as the following Expression (17).

$\begin{matrix}{{f_{2} - f_{1}} = {{\frac{N}{R}{fr}} + {2{fm}}}} & (17)\end{matrix}$

Expression (17) represents an offset frequency f₂−f₁ between the firstfrequency f₁ included in the first light oscillated by the first lightsource 11 and the second frequency f₂ included in the second lightoscillated by the second light source 12. The offset frequency f₂−f₁ isdetermined by the first frequency division number N, the secondfrequency division number R, the frequency fr included in the referencesignal and the modulation frequency fm.

The offset frequency f₂−f₁ corresponds to a frequency difference betweenthe first frequency f₁ included in the first synchronization light andthe second frequency f₂ included in the second synchronization light.The first synchronization light and the second synchronization light arelight forming offset locking light.

When Expression (16) is solved for N, the following Expression (18) isobtained.

$\begin{matrix}\begin{matrix}{N = {\frac{R}{fr}\left( {f_{2} - f_{1} - {2{fm}}} \right)}} \\{= {\frac{R}{fr}\left( {{\Delta\; f_{set}} - {2{fm}}} \right)}}\end{matrix} & (18)\end{matrix}$

Upon receiving the set value Δf_(set) of the frequency difference Δfbetween the first frequency f₁ and the second frequency f₂ from theoutside of the device, the signal processing unit 24 sets each of thefirst frequency division number N and the second frequency divisionnumber R in accordance with the set value Δf_(set) as the firstembodiment.

The signal processing unit 24 outputs each of the first frequencydivision number N and the second frequency division number R to the PLLcircuit 22.

Similarly to the first embodiment, the signal processing unit 24 setseach of the first control signal for controlling the first frequency f₁and the second control signal for controlling the second frequency f₂ inaccordance with the set value Δf_(set).

The signal processing unit 24 outputs the first control signal to thefirst light source 11 and outputs the second control signal to secondlight source 12.

Similarly to the first embodiment, the first light source 11 changes thefirst frequency f₁ included in the first light in accordance with thefirst control signal output from the signal processing unit 24, andoscillates the first light including the changed first frequency f₁.

Thus, the first frequency f₁ included in the first light is adjusted inaccordance with the set value Δf_(set) of the frequency difference Δf.

The first light source 11 outputs the first light including the firstfrequency f₁ to the first optical distributor 13.

Similarly to the first embodiment, the second light source 12 changesthe second frequency f₂ included in the second light in accordance witheach of the phase error signal output from the loop filter 23 of thelight source control circuit 21 and the second control signal outputfrom the signal processing unit 24, and oscillates the second lightincluding the changed second frequency f₂.

Thus, the second frequency f₂ included in the second light is adjustedin accordance with the set value Δf_(set) of the frequency differenceΔf.

The second light source 12 outputs the second light including the secondfrequency f₂ to the second optical distributor 14.

When receiving the first light from the first light source 11, the firstoptical distributor 13 distributes the first light and outputs the firstsplit light, which is one light after the distribution, to the LN phasemodulator 17.

The first optical distributor 13 outputs the first synchronizationlight, which is the other light after the distribution, to the outsideof the device as one light of the offset locking light.

When receiving the second light from the second light source 12, thesecond optical distributor 14 distributes the second light and outputsthe second split light, which is one light after the distribution, tothe LN phase modulator 157.

The second optical distributor 14 outputs the second synchronizationlight, which is the other light after the distribution, to the outsideof the device as the other light in the offset locking light.

FIG. 24 is an explanatory diagram illustrating offset locking light.

In FIG. 24 , the frequency f₁ is the first frequency included in thefirst synchronization light, and the frequency f₂ is the secondfrequency included in the second synchronization light.

Since each of the first frequency f₁ and the second frequency f₂ isadjusted in accordance with the set value Δf_(set) of the frequencydifference Δf, the frequency difference Δf between the first frequencyf₁ and the second frequency f₂ can be changed.

In the seventh embodiment described above, the detection circuit 156modulates the second light oscillated by the second light source 12 bythe local oscillation signal oscillated by the first local oscillationsignal source 15. The optical frequency control device 1 illustrated inFIG. 22 is configured in such a manner that the detection circuit 156detects the differential beat signal including the differentialfrequency between the frequency of the sideband light included in themodulated first light and the frequency of the sideband light includedin the modulated second light instead of detecting the differential beatsignal including the differential frequency between the frequency of thesideband light included in the modulated first light and the secondfrequency. Therefore, similarly to the optical frequency control device1 illustrated in FIG. 1 , the optical frequency control device 1illustrated in FIG. 22 can change the frequency difference between thefirst frequency included in the first light and the second frequencyincluded in the second light without using a frequency synthesizer.

In the optical frequency control device 1 illustrated in FIG. 22 , thephotodiode 159 detects, from the electric signal, a differentialfrequency (f₂−f₁)−2fm between the frequency f₁+fm of the sideband lightincluded in the modulated first light and the frequency f₂−fm of thesideband light included in the modulated second light. However, this ismerely an example, and for example, as shown in FIG. 25 , the photodiode159 may detect, from the electric signal, the differential frequency(f₂−f₁)−4fm between the frequency f₁+2fm of the sideband light includedin the modulated first light and the frequency f₂−2fm of the sidebandlight included in the modulated second light. In this case, since thephotodiode 159 outputs the signal including the differential frequency(f₂−f₁)−4fm to the PLL circuit 22 as the differential beat signal, theoffset locking light is as illustrated in FIG. 26 .

FIG. 25 is an explanatory diagram illustrating a first frequency f₁included in first light, frequencies f₁−fm, f₁+fm, f₁−2fm, f₁+2fm ofsideband light included in the modulated first light, a second frequencyf₂ included in second light, and frequencies f₂−fm, f₂+fm, f₂−2fm,f₂+2fm of sideband light included in the modulated second light.

In FIG. 25 , (f₂−2fm)−(f₁+2fm)=(f₂−f₁)−4fm is a differential frequencybetween the frequency f₁+2fm of the sideband light included in themodulated first light and the frequency f₂−2fm of the sideband lightincluded in the modulated second light.

FIG. 26 is an explanatory diagram illustrating offset locking light.

In FIG. 26 , the frequency f₁ is the first frequency included in thefirst synchronization light, and the frequency f₂ is the secondfrequency included in the second synchronization light.

Eighth Embodiment

In an eighth embodiment, an optical oscillation device 2 will bedescribed in which a light source control circuit 21 outputs a phaseerror signal to a first local oscillation signal source 15 a, and thefirst local oscillation signal source 15 a adjusts the frequency of theoscillating local oscillation signal in accordance with the phase errorsignal output from the light source control circuit 21.

FIG. 27 is a configuration diagram illustrating the optical oscillationdevice 2 including an optical frequency control device 1 according tothe eighth embodiment. In FIG. 27 , the same reference signs as those inFIGS. 1, 8, 12, 16, 19, and 22 denote the same or corresponding parts,and thus the description thereof is omitted.

In the eighth embodiment, a loop filter 23 of the light source controlcircuit 21 outputs a phase error signal to each of a second light source12 and a first local oscillation signal source 15 a.

When the phase error signal output from the light source control circuit21 is 0, the first local oscillation signal source 15 a oscillates alocal oscillation signal having a frequency of fm similarly to the firstlocal oscillation signal source 15 illustrated in FIG. 1 .

The first local oscillation signal source 15 a is connected to an LNphase modulator 17 via, for example, an optical fiber, and outputs alocal oscillation signal to the LN phase modulator 17.

The first local oscillation signal source 15 a adjusts the frequency fmof the oscillating local oscillation signal in accordance with the phaseerror signal output from the light source control circuit 21.

In the optical oscillation device 2 illustrated in FIG. 27 , the firstlocal oscillation signal source 15 a is applied to the opticaloscillation device 2 illustrated in FIG. 1 . However, this is merely anexample, and the first local oscillation signal source 15 a may beapplied to the optical oscillation device 2 illustrated in FIG. 8, 12,16, 19 , or 22.

Next, the operation of the optical oscillation device 2 illustrated inFIG. 27 will be described.

Since the components other than the first local oscillation signalsource 15 a are similar to those of the optical oscillation device 2illustrated in FIG. 1 , the operation of the first local oscillationsignal source 15 a will be mainly described herein.

When the phase error signal output from the light source control circuit21 is 0, the first local oscillation signal source 15 a maintains thefrequency fm of the oscillating local oscillation signal.

When the phase error signal indicates that the phase of the first lightis advanced from the phase of the second light, the first localoscillation signal source 15 a adjusts the frequency fm of the localoscillation signal to be small.

When the phase error signal indicates that the phase of the first lightis delayed from the phase of the second light, the first localoscillation signal source 15 a adjusts the frequency fm of the localoscillation signal to be large.

The LN phase modulator 17 modulates the first split light output fromthe first optical distributor 13 by the local oscillation signaloscillated by the first local oscillation signal source 15 a, therebygenerating sideband light having a frequency of f₁±fm.

If the frequency fm of the local oscillation signal is adjusted to besmall by the first local oscillation signal source 15 a, as illustratedin FIG. 28A, the frequency difference between the frequency f₁−fm andthe frequency f₁+fm included in the sideband light generated by the LNphase modulator 17 becomes small.

If the frequency fm of the local oscillation signal is adjusted to belarge by the first local oscillation signal source 15 a, as illustratedin FIG. 28B, the frequency difference between the frequency f₁−fm andthe frequency f₁+fm included in the sideband light generated by the LNphase modulator 17 becomes large.

FIG. 28A is an explanatory diagram illustrating a change in the sidebandlight when the frequency fm of the local oscillation signal is adjustedto be small. FIG. 28B is an explanatory diagram illustrating a change inthe sideband light when the frequency fm of the local oscillation signalis adjusted to be large.

When the first local oscillation signal source 15 a adjusts thefrequency fm of the local oscillation signal in accordance with thephase error signal, the frequency f₂−(f₁+fm) of the difference includedin the differential beat signal detected by the detection circuit 16changes.

As the frequency f₂−(f₁+fm) of the difference included in thedifferential beat signal changes, in the example of FIG. 28A, the secondfrequency f₂ included in the second light changes in a direction ofdecreasing.

As the frequency f₂−(f₁+fm) of the difference included in thedifferential beat signal changes, in the example of FIG. 28B, the secondfrequency f₂ included in the second light changes in a direction ofincreasing.

Ninth Embodiment

In a ninth embodiment, a frequency conversion device including theoptical oscillation device 2 according to any one of the first to eighthembodiments will be described.

FIG. 29 is a configuration diagram illustrating the frequency conversiondevice according to the ninth embodiment.

An optical oscillation device 2 is the optical oscillation deviceaccording to any one of the first to eighth embodiments.

The optical oscillation device 2 outputs first synchronization light,which is one light in the offset locking light, to an optical modulator131 described later.

The optical oscillation device 2 outputs the second synchronizationlight, which is the other light of the offset locking light, to theoptical multiplexer 132 described later.

The optical modulator 131 is implemented by, for example, an LN phasemodulator.

The optical modulator 131 modulates the first synchronization lightoutput from the optical oscillation device 2 with a microwave inputsignal or a millimeter wave input signal, and outputs the modulatedfirst synchronization light to the optical multiplexer 132.

In the frequency conversion device illustrated in FIG. 29 , the opticalmodulator 131 is implemented by an LN phase modulator. However, this ismerely an example, and the optical modulator 131 may be implemented by,for example, a Mach-Zehnder type intensity modulator or anelectro-absorption type modulator.

The optical multiplexer 132 multiplexes the second synchronization lightoutput from the optical oscillation device 2 and the modulated firstsynchronization light output from the optical modulator 131, and outputsthe multiplexed light, which is the light after being multiplexed, to anoptical power stabilizer 133 described later.

The optical power stabilizer 133 suppresses fluctuation in the opticalpower of the multiplexed light output from the optical multiplexer 132.

The optical power stabilizer 133 includes, for example, a monitoroptical coupler, a photodiode, an analog-to-digital converter(Hereinafter, referred to as an “AD converter”), and a semiconductoroptical amplifier or an optical variable attenuator. The optical powerstabilizer 133 monitors the optical power of the multiplexed lightoutput from the optical multiplexer 132 using the monitor opticalcoupler, the photodiode, and the AD converter. The optical powerstabilizer 133 performs gain adjustment of the semiconductor opticalamplifier or gain adjustment of the optical variable attenuator inaccordance with the monitoring result of the optical power, therebysuppressing fluctuation of the optical power of the multiplexed light.

A photoelectric converter 134 is implemented by, for example, aphotodiode.

The photoelectric converter 134 converts the multiplexed light, in whichthe fluctuation of the optical power is suppressed by the optical powerstabilizer 133, into an electric signal.

The photoelectric converter 134 outputs the electric signal to theoutside of the device as a frequency conversion output signal.

Next, the operation of the frequency conversion device illustrated inFIG. 29 will be described.

The optical oscillation device 2 outputs the first synchronization lightincluding the first frequency f₁ to the optical modulator 131 andoutputs the second synchronization light including the second frequencyf₂ to the optical multiplexer 132.

Upon receiving the first synchronization light from the opticaloscillation device 2, the optical modulator 131 modulates the firstsynchronization light with a microwave input signal or a millimeter waveinput signal.

The optical modulator 131 outputs the modulated first synchronizationlight to the optical multiplexer 132.

The optical multiplexer 132 multiplexes the second synchronization lightoutput from the optical oscillation device 2 and the modulated firstsynchronization light output from the optical modulator 131.

The optical multiplexer 132 outputs the multiplexed light, which islight after being multiplexed, to the optical power stabilizer 133.

The frequency of the multiplexed light is a frequency obtained byup-converting or down-converting the frequency of the microwave inputsignal or the frequency of the millimeter wave input signal by theoffset frequency f₂−f₁.

The optical power stabilizer 133 suppresses fluctuation in the opticalpower of the multiplexed light output from the optical multiplexer 132.

Since the fluctuation of the optical power of the multiplexed light issuppressed by the optical power stabilizer 133, it is possible tocorrect the fluctuation of the optical power of the multiplexed lightgenerated when the optical frequency control device 1 changes the offsetfrequency f₂−f₁, or the fluctuation of the optical power of themultiplexed light generated when the optical phase synchronizationfollows the wavelength fluctuation in each of the first light source 11and the second light source 12. By correcting the fluctuation in theoptical power of the multiplexed light, the fluctuation in the frequencyconversion efficiency can be suppressed.

The photoelectric converter 134 converts the multiplexed light, in whichthe optical power is stabilized by the optical power stabilizer 133,into an electric signal, and outputs the electric signal to the outsideof the device as a frequency conversion output signal.

In the ninth embodiment described above, the frequency conversion deviceis configured to include the optical oscillation device 2, the opticalmodulator 131 that modulates the first light oscillated by the firstlight source 11 by the input signal, the optical multiplexer 132 thatmultiplexes the second light oscillated by the second light source 12and the first light modulated by the optical modulator 131, and thephotoelectric converter 134 that converts the light after multiplexed bythe optical multiplexer 132 into an electric signal. Therefore, thefrequency conversion device can up-convert or down-convert the frequencyof the microwave input signal or the frequency of the millimeter waveinput signal by the offset frequency f₂−f₁.

Tenth Embodiment

in a tenth embodiment, a radio wave generation device including theoptical oscillation device 2 according to any one of the first to eighthembodiments.

FIG. 30 is a configuration diagram illustrating the radio wavegeneration device according to the tenth embodiment. In FIG. 30 , thesame reference signs as those in FIGS. 1 and 29 denote the same orcorresponding parts, and thus the description thereof is omitted.

An optical oscillation device 2 is the optical oscillation deviceaccording to any one of the first to eighth embodiments.

The optical oscillation device 2 outputs first synchronization light,which is one light of offset locking light, to an optical multiplexer141 described later.

The optical oscillation device 2 outputs second synchronization light,which is the other light of the offset locking light, to the opticalmultiplexer 141.

The optical multiplexer 141 multiplexes the first synchronization lightoutput from the optical oscillation device 2 and the secondsynchronization light output from the optical oscillation device 2.

The optical multiplexer 141 outputs the light, which is multiplexed, tothe optical power stabilizer 133.

The frequency of the multiplexed light is the offset frequency f₂−f₁.

Next, the operation of the radio wave generation device illustrated inFIG. 30 will be described.

The optical oscillation device 2 outputs the first synchronization lightincluding the first frequency f₁ to the optical multiplexer 141, andoutputs the second synchronization light including the second frequencyf₂ to the optical multiplexer 141.

The optical multiplexer 141 multiplexes the first synchronization lightoutput from the optical oscillation device 2 and the secondsynchronization light output from the optical oscillation device 2.

The optical multiplexer 141 outputs the multiplexed light of the firstsynchronization light and the second synchronization light to theoptical power stabilizer 133.

The optical power stabilizer 133 suppresses fluctuation in the opticalpower of the multiplexed light output from the optical multiplexer 141.

A photoelectric converter 134 converts the multiplexed light, in whichthe optical power is stabilized by the optical power stabilizer 133,into an electric signal, and outputs the electric signal to the outsideof the device as a microwave signal or a millimeter wave signal.

The microwave signal or the millimeter wave signal is a signal includingthe offset frequency f₂−f₁.

In the tenth embodiment described above, the radio wave generationdevice is configured to include the optical oscillation device 2, theoptical multiplexer 141 that multiplexes the first light oscillated bythe first light source 11 and the second light oscillated by the secondlight source 12, and the photoelectric converter 134 that converts thelight multiplexed by the optical multiplexer 141 into an electricsignal. Therefore, the radio wave generation device can output a signalincluding the offset frequency f₂−f₁.

Note that, in the scope of the present invention, the present inventionof this application allows free combinations of each embodiment,modification of any constituents of each embodiment, or omission of anyconstituents in each embodiment.

INDUSTRIAL APPLICABILITY

The present invention is suitable for an optical frequency controldevice that changes a frequency included in light, an opticaloscillation device including the optical frequency control device, afrequency conversion device including the optical oscillation device,and a radio wave generation device including the optical oscillationdevice.

REFERENCE SIGNS LIST

1: optical frequency control device, 2: optical oscillation device, 11:first light source, 12: second light source, 13: first opticaldistributor, 14: second optical distributor, 15, 15 a: first localoscillation signal source, 16: detection circuit, 17: LN phasemodulator, 18: optical multiplexer, 19: photodiode, 20: reference signalsource, 21: light source control circuit, 22: PLL circuit, 23: loopfilter, 24: signal processing unit, 31: laser diode, 32: constantcurrent driver, 33: thermistor, 34: TEC driver, 35: Peltier element, 41:laser diode, 42: constant current driver, 43: thermistor, 44: TECdriver, 45: Peltier element, 51: prescaler, 52: prescaler, 53: phasecomparator, 54: polarity switch, 61: frequency division number settingunit, 61 a: subtractor, 61 b: divider, 61 c: multiplier, 62: controlsignal setting unit, 62 a: table, 71: voltage monitor. 72: signalprocessing unit, 73: voltage range storing unit, 74: comparator, 75:control signal setting unit, 81: signal processing unit, 82: frequencydivision number setting unit, 82 a: subtractor, 82 b: comparator. 82 c:increment unit, 82 d: multiplier, 82 e: divider, 82 f: multiplier, 90:light source control circuit, 91: PLL circuit, 100: signal processingunit, 101: frequency division number setting unit, 101 a: subtractor,101 b: comparator, 101 c: increment unit, 101 d: multiplier, 101 e:absolute value calculator, 101 f: divider, 101 g: multiplier, 101 h:sign extractor, 110: power regulator, 120: signal processing unit, 121:power calculator, 131: optical modulator, 132: optical multiplexer, 133:optical power stabilizer, 134: photoelectric converter, 141: opticalmultiplexer, 151: second local oscillation signal source, 152: detectioncircuit, 153: LN phase modulator, 154: optical multiplexer, 155:photodiode, 156: detection circuit, 157: LN phase modulator, 158:optical multiplexer, 159: photodiode

The invention claimed is:
 1. An optical frequency control device,comprising: a detection circuit to receive first light including a firstfrequency from a first light source, receive second light including asecond frequency from a second light source, modulate the first lightwith a local oscillation signal oscillated by a first local oscillationsignal source, and detect a differential beat signal including adifferential frequency between a frequency of sideband light included inthe first light, which is modulated, and the second frequency; a lightsource control circuit to change the second frequency included in thesecond light oscillated by the second light source by frequency-dividinga differential beat signal detected by the detection circuit with afirst frequency division number, by frequency-dividing a referencesignal oscillated by a reference signal source with a second frequencydivision number, and by outputting, to the second light source, a phaseerror signal, which indicates a phase difference between thedifferential beat signal after the frequency division and the referencesignal after the frequency division number; and a signal processor toset each of a first frequency division number and a second frequencydivision number in accordance with a set value of a frequency differencebetween the first frequency division number and the second frequencydivision number.
 2. The optical frequency control device according toclaim 1, wherein the signal processor determines an order of thesideband light on a basis of the set value and sets the first frequencydivision number in accordance with each of the set value and the order.3. The optical frequency control device according to claim 1, whereinthe signal processor switches a polarity of a phase error signal outputfrom the light source control circuit.
 4. The optical frequency controldevice according to claim 2, further comprising a power regulator toregulate power of a local oscillation signal oscillated by the firstlocal oscillation signal source, wherein the signal processor controlsregulation of power by the power regulator on a basis of an order of thesideband light.
 5. The optical frequency control device according toclaim 1, wherein the detection circuit modulates the second lightoscillated by the second light source with a local oscillation signaloscillated by a second local oscillation signal source, and detects adifferential beat signal including a differential frequency between afrequency of sideband light included in the first light after modulationand a frequency of sideband light included in the second light aftermodulation, instead of detecting a differential beat signal including adifferential frequency between the frequency of the sideband light andthe second frequency.
 6. The optical frequency control device accordingto claim 1, wherein the detection circuit modulates the second lightoscillated by the second light source with a local oscillation signaloscillated by a first local oscillation signal source, and detects adifferential beat signal including a differential frequency between afrequency of sideband light included in the first light after modulationand a frequency of sideband light included in the second light aftermodulation, instead of detecting a differential beat signal including adifferential frequency between the frequency of the sideband light andthe second frequency.
 7. The optical frequency control device accordingto claim 1, wherein the light source control circuit regulates afrequency of a local oscillation signal oscillated by the first localoscillation signal source by outputting the phase error signal to thefirst local oscillation signal source.
 8. An optical oscillation devicecomprising: the optical frequency control device according to claim 1; afirst light source to oscillate first light; a second light source tooscillate second light; a first local oscillation signal source tooscillate a local oscillation signal; and a reference signal source tooscillate a reference signal.
 9. The optical oscillation deviceaccording to claim 8, wherein the signal processor outputs a firstcontrol signal for controlling a first frequency included in first lightoscillated by the first light source and a second control signal forcontrolling a second frequency included in second light oscillated bythe second light source in accordance with the set value, the firstlight source changes a first frequency included in the first light inaccordance with the first control signal output from the signalprocessor, and the second light source changes a second frequencyincluded in the second light in accordance with each of a phase errorsignal output from the light source control circuit and the secondcontrol signal output from the signal processor.
 10. The opticaloscillation device according to claim 9, wherein the signal processorregulates each of the first control signal and the second control signalon a basis of a phase error signal output from the light source controlcircuit.
 11. The optical oscillation device according to claim 8,further comprising: a first optical distributor to distribute the lightoscillated by the first light source, output one light after thedistribution to the detection circuit, and output another light afterthe distribution as one light in offset locking light; and a secondoptical distributor to distribute the light oscillated by the secondlight source, to output one light after the distribution to thedetection circuit, and to output the other light after the distributionas another light in the offset locking light.
 12. The opticaloscillation device according to claim 8, further comprising a secondlocal oscillation signal source to oscillate a local oscillation signal,wherein the detection circuit modulates the second light oscillated bythe second light source with a local oscillation signal oscillated bythe second local oscillation signal source, and detects a differentialbeat signal including a differential frequency between a frequency ofsideband light included in the first light after modulation and afrequency of sideband light included in the second light aftermodulation, instead of detecting a differential beat signal including adifferential frequency between the frequency of the sideband light andthe second frequency.
 13. The optical oscillation device according toclaim 8, wherein the detection circuit modulates the second lightoscillated by the second light source with a local oscillation signaloscillated by the first local oscillation signal source, and detects adifferential beat signal including a differential frequency between afrequency of sideband light included in the first light after modulationand a frequency of sideband light included in the second light aftermodulation, instead of detecting a differential beat signal including adifferential frequency between the frequency of the sideband light andthe second frequency.
 14. The optical oscillation device according toclaim 8, wherein the light source control circuit outputs the phaseerror signal to the first local oscillation signal source, and the firstlocal oscillation signal source regulates a frequency of an oscillatinglocal oscillation signal in accordance with the phase error signaloutput from the light source control circuit.
 15. The opticaloscillation device according to claim 8, wherein one or more of thefirst light source, the second light source, and the detection circuitare integrated as a planar lightwave circuit.
 16. The opticaloscillation device according to claim 8, wherein one or more of thefirst light source, the second light source, the detection circuit, thelight source control circuit, and the signal processor are integratedusing silicon.
 17. A frequency conversion device comprising: the opticaloscillation device according to claim 8; an optical modulator tomodulate first light oscillated by the first light source with an inputsignal; an optical multiplexer to multiplex the second light oscillatedby the second light source and the light modulated by the opticalmodulator; and a photoelectric converter to convert light multiplexed bythe optical multiplexer into an electric signal.
 18. The frequencyconversion device according to claim 17, further comprising an opticalpower stabilizer to suppress power fluctuation of light multiplexed bythe optical multiplexer and output light after the power fluctuationsuppression to the photoelectric converter.
 19. A radio wave generationdevice comprising: the optical oscillation device according to claim 8;an optical multiplexer to multiplex first light oscillated by the firstlight source and second light oscillated by the second light source; anda photoelectric converter to convert light multiplexed by the opticalmultiplexer into an electric signal.
 20. The radio wave generationdevice according to claim 19, further comprising an optical powerstabilizer to suppress power fluctuation of light multiplexed by theoptical multiplexer and output the light after the power fluctuationsuppression to the photoelectric converter.